AR15.Com Archives
 20" Barrel 1-7 Twist - Dissadvantages???
Blain  [Member]
10/4/2010 6:18:50 PM EST
On my 20" A2 rifle build, I have a choice of going with either a GI profile barrel in 1-7 twist, or the standard heavy 1-9 twist barrel (both would be chrome lined).

At first I instantly thought the GI profile was the way to go. Slimmer profile means less weight and faster cooling. 1-7 twist means I can shoot the heavier bullets and can give my AR more effective range. Then I started to think about the potential disadvantages....

1-7 twist over stabilizes the cheaper and more readily available 55 grain M193 surplus. I have heard that this could make the bullet less likely to fragment / tumble upon impacting a human target as it is more stable than out of a slower twist rate. M193 is known as a very effective stopper due to it's ability to explosively fragment and tumble. I don't want to potentially take that away, esp when most SHTF stockpiles are that or greentip.

I have also read that 1-7 twist rates wear the barrel excessively faster. I recall reading about a test that the marines did in the early 80s that showed a 1-7 twist rate caused the barrel to wear substantially after only 6,000 - 7,000 rounds.

Also, the heavier profile of the 1-9 twist barrel might sustain better accuracy for longer as it is thicker, much like a bull barrel.

Is a 1-7 twist needed in a 20" barrel? 1-9 twist will give a little more than two full rotations in a 20" barrel. A 1-7 twist gives a little more than two full rotations in a 16" barrel, and that is all that is needed to shoot the 75 and 77 grain bullets out of a 16" barrel.

What do you guys think? I'm looking for advice here, to see the advantages / disadvantages of each, and to see what you prefer and why.

Thank you.
Paid Advertisement
--
HeavyMetal  [Team Member]
10/4/2010 6:20:22 PM EST
1/7 does not over stabilize the 55 grain stuff and it does not make it any less likely to fragment. It shoots fine in a 1.7 twist.

The 1/9 may or may not stabilize the 77 gr stuff in the colder weather. Plus, unless you are going to spend all your time shooting this off of a bench, a full length H-Bar just plain sucks for dead weight to carry.
Molon  [Team Member]
10/4/2010 6:30:48 PM EST
Originally Posted By Blain:

Then I started to think about the potential disadvantages....

1-7 twist over stabilizes the cheaper and more readily available 55 grain M193 surplus. I have heard that this could make the bullet less likely to fragment / tumble upon impacting a human target as it is more stable than out of a slower twist rate. M193 is known as a very effective stopper due to it's ability to explosively fragment and tumble. I don't want to potentially take that away, esp when most SHTF stockpiles are that or greentip.





You’re posting a lot of false information in a technical forum. Please spend some time researching the resources available on this website. Here's some info to get you started.



from Dr. G.K. Roberts . . .

"The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists. Similar testing was conducted with 5.56 mm 62 gr M855 FMJ ammunition fired in 1/9 and 1/7 twist barrels. Again, no difference in terminal performance was noted."


From the Ammo Oracle.







M193 is a poor choice as a self-defense round compared to the numerous modern loads available today.

From Dr. G.K. Roberts . . .


5.56 mm Duty Loads


For LE Patrol use, where there is a high incidence of potential engagements around or involving vehicles, ammunition that is able to effectively penetrate intermediate barriers, particularly vehicle glass is critical. The best LE 5.56 mm/.223 loads for intermediate barrier penetration using 1/9 and faster twist barrels are the 55 & 62 gr Federal bonded JSP Tactical loads (LE223T1 & LE223T3), along with the Nosler 60 gr Partition JSP, Remington 62 gr bonded JSP, Speer 55 & 64 gr Gold Dot JSP's (and identically constructed Federal 62 gr Fusion JSP); the Swift 75 gr Scirrocco PT is also good choice, but usually requires a 1/7 twist. The Barnes all copper TSX bullets are great projectiles (the 55 gr TSX is optimal for LE use––ex. Fed T223S)) and offer good penetration through barriers, however, when first hitting a laminated automobile windshield intermediate barrier, TSX bullets exhibit less expansion than bonded JSP’s, as the Barnes jacket either collapses at the nose, the jacket "petals" fold back against the core, or the "petals" are torn off; this results in a caliber size projectile configured a lot like a full wadcutter, leading to deep penetration.

If running 1/12 twist barrels, stick with the 55 gr Gold Dot and 55 gr TSX. If available, the Federal 62 gr Mk318 Mod0 (T556TNB1) is another good load for LE use in 1/9 twist and faster barrels. None of the 5.56 mm OTM bullets, even the heavy 75 - 100 gr loads, offer acceptable performance through automobile windshield glass. Contrary to what many believe, M193 & M855 FMJ are not very good against glass. The best military 5.56 mm load against glass is 52 gr M995 AP, followed by the 62 gr Mk318 Mod0 and 70 gr Optimal "brown tip"

In those situations where intermediate barrier penetration is not a critical requirement, for example LE urban entries or long range shots in open conditions, then OTM, JHP, and standard JSP loads can offer good performance. If your expected engagement scenarios are at typical LE distances, say out to 200 yards, then either 5.56 mm or .223 SAAMI pressure loads are fine. For 1/7 twist barrels, the Hornady 75 gr OTM, Nosler 77 gr OTM, and Sierra 77 gr SMK OTM are all good choices. The experimental BH loaded 100 gr OTM exhibits impressive fragmentation, even at relatively low velocities, however while capable of shooting out to 600, it is optimized for 200 and under.

If stuck with 1/9 twist barrels, the heavy 70+ gr loads are not universally accurate in all rifles and the 69 gr SMK OTM, the 68 gr Hornady OTM, the Winchester 64 gr JSP (RA223R2), the Federal 64 gr TRU (T223L) JSP, Hornady 60 gr JSP, are likely to run accurately in the majority of 1/9 twist rifles.

For longer range precision weapons (like the Mk12) with faster 1/8 or 1/7 twist barrels I would choose one of the combat proven 5.56 mm (ie. 5.56 mm NATO pressure loads, not .223 SAAMI pressure loads which run about 200 f/s slower) heavy match OTM loadings: either the Hornady 75 gr TAP (#8126N) using the OTM bullet w/cannelure or the equally good 77 gr Nosler OTM w/cannelure loaded by Black Hills, followed by the 77 gr Sierra Match King OTM––which, while exceedingly accurate, offers slightly reduced terminal effects.


1:7" twist versus 1:9" twist with 55 grain FMJ Ammunition






In this article, we’ll be taking a look at the actual level of accuracy (technically precision) that is consistently achievable with the ubiquitous Wolf Performance Ammunition when fired from a chrome-lined, NATO chambered, semi-automatic AR-15. The “Performance Ammunition” evaluated for this article is loaded in polymer-coated steel cases and uses full metal jacket bullets with “bimetal” jackets; a three-layered affair consisting of a very thin outer layer of copper, a thick middle layer of steel and another thin layer of copper forming the inside of the jacket.



This accuracy evaluation used statistically significant shot-group sizes and every single shot in a fired group was included in the measurements. There was absolutely no use of any Group Reduction Techniques (e.g. fliers, target movement, Butterfly Shots). The shooting set-up will be described in detail below. As many of the significant variables as was practicable were controlled for. Also, "control groups" were fired from the barrels used in the evaluation using match-grade hand-loaded ammunition; in order to demonstrate the capability of each barrel. Pictures of the fired shot-groups will be posted for documentation.

All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) The barrels used in the evaluation were free-floated. The free-float handguards of the rifles rested in a Sinclair Windage Benchrest, while the stock of the rifles rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The set-up was very similar to that pictured below.






The Wind Probe.





The first Wolf Performance load evaluated was the 55 grain FMJ version. The test vehicle was a 20” Colt HBAR with a 1:7” twist, chrome-lining and a NATO chamber. A 10-shot control load was fired from this barrel at 100 yards using hand-loaded Sierra MatchKings. The extreme spread of that group was 1.03”.










Four 10-shot groups of the 55 grain FMJ Wolf ammunition were fired in a row from 100 yards with the resulting extreme spreads:

4.22”
3.73”
3.34”
4.31”

for a 10-shot group average extreme spread of 3.9”. All four of the 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius for the composite group was 1.16”.


The “smallest” 10-shot group.






The 40-shot composite group.





Knowing that the Internet Commando would feel that the above evaluation was unfair, due to his false belief that a 1:7” twist barrel “over-stabilizes” 55 grain bullets, I repeated the entire evaluation using a 1:9” twist barrel. The results were nearly identical.

The new test vehicle was a 16” Colt HBAR with chrome lining, a NATO chamber and of course the 1:9” twist. This is the barrel found on the Colt 6721 carbine. This barrel was also free-floated. The exact same lot of Wolf 55 grain FMJ ammunition was used for this repeated evaluation. All other aspects of the evaluation were exactly as described above for the previous evaluation.





A 10-shot control group fired from the 16” Colt HBAR from 100 yards using the same match-grade hand-load of Sierra 55 grain BlitzKings had an extreme spread of 0.89”.


Just as before, four 10-shot groups of the 55 grain FMJ Wolf ammunition were fired in a row from 100 yards. The extreme spread of those groups measured:

4.30”
3.62”
4.04”
3.99”

for a 10-shot group average extreme spread of 4.0”. As before, all four of the 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. This composite group had a mean radius of 1.21”.



The “smallest” group from the 1:9” twist barrel.






The 40-shot composite group from the 1:9” twist barrel.




Here are both of the 40-shot composite groups side by side for comparison.








As previously posted, the exact same conclusions were reached when performing the same test using Prvi M193 ammunition. Using the Prvi M193 ammunition, I did an accuracy comparison firing the 55 grain FMJ load from both a 1:9” twist Colt HBAR and a 1:7” twist Colt HBAR. Four 10-shot groups were fired from each barrel from a bench-rest at a distance of 100 yards. The groups from each barrel were over-layed to form 40-shot composite groups. The mean radii of the composite groups were nearly identical.







Quality 55 grain bullets can shoot superbly from fast twist barrels. Most 55 grain full metal jacket bullets do not fall into the quality category. The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist.





The 3-shot group pictured below was fired from the same 1:7” twist barrel for the Internet Commandos in our viewing audience.







The next 10-shot group pictured was fired from a Krieger barreled AR-15 with a 1:7.7” twist using 55 grain BlitzKings.




Barrels with 1:7" twists can shoot bullets as light in weight as 40 grains without issue, as long as it is a modern, quality 40 grain bullet, such as the Hornady 40 grain V-MAX.








Here's another example of just how well a 1:7" twist barrel can shoot light weight bullets. While the group pictured below was fired from a distance of only 50 yards, it's a 10-shot group fired from a chrome lined, NATO chambered Colt barrel with a 1:7" twist using 52 grain Sierra MatchKings.








A 14.5" Colt M4A1 SOCOM barrel,with its NATO chamber, chrome lining and 1:7" twist can shoot quality 55 grain bullets quite well.







According to C.E. Harris, testing at Aberdeen Proving Ground has shown that “For military use you generally want Sg (the gyroscopic stability factor) to be in the range of 1.5 - 2.0 . . .”

For the M855 bullet, a gyroscopic stability factor of approximately 1.5 (depending upon velocity) will be obtained with a twist rate of 1:8.3”.







For the M855 bullet, a gyroscopic stability factor of approximately 2.0 (depending upon velocity) will be obtained with a twist rate of 1:7.2”.






Additionaly C.E. Harris reported that testing has shown that “bullets which are well balanced, more than adequate twist does not hurt until you get to a gyroscopic stability factor greater than 5.0.” To put that into perspective, a Hornady 55 grain FMJ bullet would have to be fired from a barrel with a 1:6” twist rate at a muzzle velocity of 4000 fps to obtain a gyroscopic stability factor greater than 5.0.



....
Fields_Overseer  [Member]
10/4/2010 6:42:24 PM EST
Originally Posted By Blain:
On my 20" A2 rifle build, I have a choice of going with either a GI profile barrel in 1-7 twist, or the standard heavy 1-9 twist barrel (both would be chrome lined).

At first I instantly thought the GI profile was the way to go. Slimmer profile means less weight and faster cooling. 1-7 twist means I can shoot the heavier bullets and can give my AR more effective range. Then I started to think about the potential disadvantages....

1-7 twist over stabilizes the cheaper and more readily available 55 grain M193 surplus. I have heard that this could make the bullet less likely to fragment / tumble upon impacting a human target as it is more stable than out of a slower twist rate. M193 is known as a very effective stopper due to it's ability to explosively fragment and tumble. I don't want to potentially take that away, esp when most SHTF stockpiles are that or greentip.

I have also read that 1-7 twist rates wear the barrel excessively faster. I recall reading about a test that the marines did in the early 80s that showed a 1-7 twist rate caused the barrel to wear substantially after only 6,000 - 7,000 rounds.

Also, the heavier profile of the 1-9 twist barrel might sustain better accuracy for longer as it is thicker, much like a bull barrel.

Is a 1-7 twist needed in a 20" barrel? 1-9 twist will give a little more than two full rotations in a 20" barrel. A 1-7 twist gives a little more than two full rotations in a 16" barrel, and that is all that is needed to shoot the 75 and 77 grain bullets out of a 16" barrel.

What do you guys think? I'm looking for advice here, to see the advantages / disadvantages of each, and to see what you prefer and why.

Thank you.


1/7 does not overstabilize 55grainers. Most people can shoot 40grainers fine from a 1/7.

I doubt weare is much different. CL barrels will last 15-20k rounds if not more.

Edit - What molon said.
Andrew9869  [Member]
10/4/2010 8:16:18 PM EST
I've shot 45grn loads out of my 1/7 bcm 20" with no issues. Don't expect two shots through a dime at 100 yards, but you can more or less shoot almost any load off the shelf in a 1/7 gun without too much trouble. I would not hesitate to go for the government barrel again, and have no reason to change. The over stabilization thing is bs especially for 55grn.
Bretshooter  [Member]
10/4/2010 9:31:02 PM EST
Originally Posted By Blain:
Also, the heavier profile of the 1-9 twist barrel might sustain better accuracy for longer as it is thicker, much like a bull barrel.


Thickness equals longevity? Are you going to be shooting this barrel full auto?
Blain  [Member]
10/5/2010 2:42:43 AM EST
So 1-7 Gov profile is the way to go? There is no benifet to the 1-9 heavy barrel?
NickySantoro  [Member]
10/5/2010 3:40:30 AM EST
OP,
I've experienced no problems with everything from 40 gr to 69 gr out of a 1:7 gov't profile barrel.
FWIW
YMMV
halfmoonclip  [Member]
10/5/2010 6:52:09 AM EST
I've had no problems with the 1:7 and 55 grs, but an HBar is another matter. Not only is it silly heavy for an AR, but all the weight is in the wrong place, making it really muzzle heavy for offhand shooting.
If you're hellfire determined to get the 1:9, you could get ADCO to turn it to Gummint Profile.
Moon
Krylancelo  [Team Member]
10/5/2010 10:20:17 AM EST
Gov't profile or lightweight profile are both fine for your usage. The only time it would become an issue is if you intend to fire very quickly or full auto. Both of these barrels (I believe) are rated to be able to fire 20 rounds per minute indefinitely without barrel failure. Are you intending to fire more than 20 shots per minute for a long period of time (several hundred rounds at least)? Then consider the HBAR. If not, then don't worry about it.

Edit: 1:7 overstabilizing or being less accurate the 1:9 is a complete total and utter myth. Don't listen to that garbage. My 1:7 is just as accurate (if not more so) than my 1:9 with 55gr ammo.
C_Rion  [Team Member]
10/5/2010 10:50:45 AM EST
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.

1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision
Krylancelo  [Team Member]
10/5/2010 11:05:42 AM EST

Originally Posted By C_Rion:
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.

1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision

Do you have copies of documentation sent between FN and the Gov or Colt and the Gov that says this? Do you know the exact reasons why they wanted 1:9 instead of 1:7? I could post on here and say that Colt convinced the Gov to go with 1:7 over 1:9 but that doesn't make it true.

Can you provide any evidence that 1:7 does a poorer job with 55-62 grain ammo than 1:9 does?

Every person that I have seen that provides evidence has clearly shown that 1:7 is not any worse than 1:9 for 55 grain ammo. My own personal experience has shown that 1:7 is no less accurate than 1:9 with 55 grain ammo.

Please substantiate your claims. Molon provided evidence from several trustworthy sources that clearly contradict what you are saying (highlighted).
C_Rion  [Team Member]
10/5/2010 12:52:15 PM EST
Originally Posted By Krylancelo:

Originally Posted By C_Rion:
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.

1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision

Do you have copies of documentation sent between FN and the Gov or Colt and the Gov that says this? Do you know the exact reasons why they wanted 1:9 instead of 1:7? I could post on here and say that Colt convinced the Gov to go with 1:7 over 1:9 but that doesn't make it true.

Can you provide any evidence that 1:7 does a poorer job with 55-62 grain ammo than 1:9 does?

Every person that I have seen that provides evidence has clearly shown that 1:7 is not any worse than 1:9 for 55 grain ammo. My own personal experience has shown that 1:7 is no less accurate than 1:9 with 55 grain ammo.

Please substantiate your claims. Molon provided evidence from several trustworthy sources that clearly contradict what you are saying (highlighted).


You can believe what ever makes you warm and fuzzy, but the above info is commonly known fact.
I don't need dosumentation between FN, Colt, and the US Govt. to know that the optimal twist for the 62 grain bullet is 1/9. The obsessive infatuation with 1/7 is an arfcom phenomenon, nothing more.

From the AR15 Ammo oracle:

The SS-109 bullet uses a "compound" core, with a lead base topped by a steel penetrator, all covered in a gilding-metal (copper alloy) jacket. The L-110 tracer bullet has a copper-plated steel jacket and like all tracer bullets, is hollowed out at the base and filled with tracing compound. Both bullets are much longer in length than the earlier 55gr bullets, especially the L-110 tracer, which was designed to trace out to 800m, verses 450m for the older M196 tracer round. Due to their increased length, these bullets require a faster rifling twist to be properly stabilized. The military settled on a twist rate of 1:7, which is a compromise between the 1:9 twist ideal for SS-109 bullets and the 1:6 twist ideal for L-110 tracers.

Barrel Twist

Forest  [Moderator]
10/5/2010 1:00:01 PM EST
Originally Posted By C_Rion:
You can believe what ever makes you warm and fuzzy, but the above info is commonly known fact.

Without supporting documentation/evidence it's not fact it's opinion.

It the Middle Ages it was 'commonly known' that sailing down to Africa would cause you to end up in a boiling sea.



I don't need dosumentation between FN, Colt, and the US Govt. to know that the optimal twist for the 62 grain bullet is 1/9.

Then you don't know if you don't have the data to support your 'knowledge'. For sure 62gr can be stabilized by 1/9, but so what? Do you get demonstrably superior accuracy, MV, or barrel longevity using 1/9 vs 1/7 (or any other twist).

In God we trust, all others must bring data.


The obsessive infatuation with 1/7 is an arfcom phenomenon, nothing more.

Well the ARFcommers like to occasionally shoot match loads which require faster than 1:9 twists for best results. Is ALL our ammo 75-80gr OTMs? Heck no, but for those that do more than shoot an occasional beer can or dirt clod having the ability to shoot heavier bullets accurately is a 'free' feature one gets by going with 1:7 over 1:9.


Krylancelo  [Team Member]
10/5/2010 1:13:50 PM EST

Originally Posted By C_Rion:
Originally Posted By Krylancelo:

Originally Posted By C_Rion:
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.

1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision

Do you have copies of documentation sent between FN and the Gov or Colt and the Gov that says this? Do you know the exact reasons why they wanted 1:9 instead of 1:7? I could post on here and say that Colt convinced the Gov to go with 1:7 over 1:9 but that doesn't make it true.

Can you provide any evidence that 1:7 does a poorer job with 55-62 grain ammo than 1:9 does?

Every person that I have seen that provides evidence has clearly shown that 1:7 is not any worse than 1:9 for 55 grain ammo. My own personal experience has shown that 1:7 is no less accurate than 1:9 with 55 grain ammo.

Please substantiate your claims. Molon provided evidence from several trustworthy sources that clearly contradict what you are saying (highlighted).


You can believe what ever makes you warm and fuzzy, but the above info is commonly known fact.
I don't need dosumentation between FN, Colt, and the US Govt. to know that the optimal twist for the 62 grain bullet is 1/9. The obsessive infatuation with 1/7 is an arfcom phenomenon, nothing more.

From the AR15 Ammo oracle:

The SS-109 bullet uses a "compound" core, with a lead base topped by a steel penetrator, all covered in a gilding-metal (copper alloy) jacket. The L-110 tracer bullet has a copper-plated steel jacket and like all tracer bullets, is hollowed out at the base and filled with tracing compound. Both bullets are much longer in length than the earlier 55gr bullets, especially the L-110 tracer, which was designed to trace out to 800m, verses 450m for the older M196 tracer round. Due to their increased length, these bullets require a faster rifling twist to be properly stabilized. The military settled on a twist rate of 1:7, which is a compromise between the 1:9 twist ideal for SS-109 bullets and the 1:6 twist ideal for L-110 tracers.

Barrel Twist


That's fine... But can you overstabilize a bullet? Isn't it either stable or unstable? What about Molon's actual data provided where it showed the 1:7 was actually MORE accurate than his 1:9 with a bullet LIGHTER than the 62gr SS-109 bullets in question? How do you explain that?

How do we know that "ideal" doesn't actually mean "minimum"? What testing was done on the 1:9 compared to the 1:7? 1:9 could have simply been the first barrel twist they tested that stabilized the 62gr SS-109 bullet and maybe they didn't even test 1:7 until they decided to due to the tracer rounds? While it says 1:9 is "ideal" it doesn't give any details about how 1:7 performs compared to 1:9, it simply says 1:7 was "settled" on. Does that mean 1:7 doesn't perform well or just as well?

What is the context/meaning of settle here?

From Dictionary.com:

set·tle
–verb (used with object)
1.
to appoint, fix, or resolve definitely and conclusively; agree upon (as time, price, or conditions).
2.
to place in a desired state or in order: to settle one's affairs.
3.
to pay, as a bill.
4.
to close (an account) by payment.
5.
to migrate to and organize (an area, territory, etc.); colonize: The pilgrims settled Plymouth.
6.
to cause to take up residence: They settled immigrants in urban areas.
7.
to furnish (a place) with inhabitants or settlers: The french settled this colony with army veterans.
8.
to quiet, calm, or bring to rest (the nerves, stomach, etc.).
9.
to stop from annoying or opposing: A sharp word will settle that youngster.
10.
to conclude or resolve: to settle a dispute.
11.
to make stable; place in a permanent position or on a permanent basis.
12.
to cause (a liquid) to become clear by depositing dregs.
13.
to cause (dregs, sediment, etc.) to sink or be deposited.
14.
to cause to sink down gradually; make firm or compact.
15.
to dispose of finally; close up: to settle an estate.
16.
Law.
a.
to secure (property, title, etc.) on or to a person by formal or legal process.
b.
to terminate (legal proceedings) by mutual consent of the parties.
–verb (used without object)
17.
to decide, arrange, or agree (often fol. by on or upon): to settle on a plan of action.
18.
to arrange matters in dispute; come to an agreement: to settle with a person.
19.
to pay a bill; make a financial arrangement (often fol. by up).
20.
to take up residence in a new country or place: Many Frenchmen settled along the Mississippi River following La Salle's explorations.
21.
to come to rest, as from flight: A bird settled on a bough.
22.
to gather, collect, or become fixed in a particular place, direction, etc.: A cold settled in my head.
23.
to become calm or composed (often fol. by down): I'll wait until the class settles before starting the lesson.
24.
to come to rest (often fol. by down): We settled down for the night at an old country inn.
25.
to sink down gradually; subside.
26.
to become clear by the sinking of suspended particles, as a liquid.
27.
to sink to the bottom, as sediment.
28.
to become firm or compact, as the ground.
29.
(of a female animal) to become pregnant; conceive.
—Verb phrases
30.
settle down,
a.
to become established in some routine, esp. upon marrying, after a period of independence or indecision.
b.
to become calm or quiet.
c.
to apply oneself to serious work: There were so many distractions that we weren't able to settle down to studying.
31.
settle for, to be satisfied with: to settle for less.
32. settle into, to become established in: to settle into a new routine.


Are we 100% sure that out of 32 definitions it means definition #31 and not definition #1 for example?

At best this is inconclusive evidence.
C_Rion  [Team Member]
10/5/2010 1:13:56 PM EST
Originally Posted By Forest:
Originally Posted By C_Rion:
You can believe what ever makes you warm and fuzzy, but the above info is commonly known fact.

Without supporting documentation/evidence it's not fact it's opinion.

It the Middle Ages it was 'commonly known' that sailing down to Africa would cause you to end up in a boiling sea.



I don't need dosumentation between FN, Colt, and the US Govt. to know that the optimal twist for the 62 grain bullet is 1/9.

Then you don't know if you don't have the data to support your 'knowledge'. For sure 62gr can be stabilized by 1/9, but so what? Do you get demonstrably superior accuracy, MV, or barrel longevity using 1/9 vs 1/7 (or any other twist).

In God we trust, all others must bring data.


The obsessive infatuation with 1/7 is an arfcom phenomenon, nothing more.

Well the ARFcommers like to occasionally shoot match loads which require faster than 1:9 twists for best results. Is ALL our ammo 75-80gr OTMs? Heck no, but for those that do more than shoot an occasional beer can or dirt clod having the ability to shoot heavier bullets accurately is a 'free' feature one gets by going with 1:7 over 1:9.




You ever hear of the Greenhill formula? Apparently not. Like I said before, I don't need documentation between FN colt and the US Govt to determine the optimal twist for 62 grain SS109 is 1/9. Mathmatic Calculations tell me all I need to know. Here, I will post a link so you will know to.

Barrel Twist Calculator
Krylancelo  [Team Member]
10/5/2010 1:22:49 PM EST

Originally Posted By C_Rion:
You ever hear of the Greenhill formula? Apparently not. Like I said before, I don't need documentation between FN colt and the US Govt to determine the optimal twist for 62 grain SS109 is 1/9. Mathmatic Calculations tell me all I need to know. Here, I will post a link so you will know to.

Barrel Twist Calculator

Howell feels one can overstablize a bullet. Ideally, the bullet's axis will keep tangent to the flight path, but overstablized, the bullet will instead remain pointing in the direction of the barrel. He offered no way to quantify such overstabilization.

Directly from the site you linked.
C_Rion  [Team Member]
10/5/2010 1:26:59 PM EST
Originally Posted By Krylancelo:

Originally Posted By C_Rion:
Originally Posted By Krylancelo:

Originally Posted By C_Rion:
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.

1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision

Do you have copies of documentation sent between FN and the Gov or Colt and the Gov that says this? Do you know the exact reasons why they wanted 1:9 instead of 1:7? I could post on here and say that Colt convinced the Gov to go with 1:7 over 1:9 but that doesn't make it true.

Can you provide any evidence that 1:7 does a poorer job with 55-62 grain ammo than 1:9 does?

Every person that I have seen that provides evidence has clearly shown that 1:7 is not any worse than 1:9 for 55 grain ammo. My own personal experience has shown that 1:7 is no less accurate than 1:9 with 55 grain ammo.

Please substantiate your claims. Molon provided evidence from several trustworthy sources that clearly contradict what you are saying (highlighted).


You can believe what ever makes you warm and fuzzy, but the above info is commonly known fact.
I don't need dosumentation between FN, Colt, and the US Govt. to know that the optimal twist for the 62 grain bullet is 1/9. The obsessive infatuation with 1/7 is an arfcom phenomenon, nothing more.

From the AR15 Ammo oracle:

The SS-109 bullet uses a "compound" core, with a lead base topped by a steel penetrator, all covered in a gilding-metal (copper alloy) jacket. The L-110 tracer bullet has a copper-plated steel jacket and like all tracer bullets, is hollowed out at the base and filled with tracing compound. Both bullets are much longer in length than the earlier 55gr bullets, especially the L-110 tracer, which was designed to trace out to 800m, verses 450m for the older M196 tracer round. Due to their increased length, these bullets require a faster rifling twist to be properly stabilized. The military settled on a twist rate of 1:7, which is a compromise between the 1:9 twist ideal for SS-109 bullets and the 1:6 twist ideal for L-110 tracers.

Barrel Twist


That's fine... But can you overstabilize a bullet? Isn't it either stable or unstable? What about Molon's actual data provided where it showed the 1:7 was actually MORE accurate than his 1:9 with a bullet LIGHTER than the 62gr SS-109 bullets in question? How do you explain that?

How do we know that "ideal" doesn't actually mean "minimum"? What testing was done on the 1:9 compared to the 1:7? 1:9 could have simply been the first barrel twist they tested that stabilized the 62gr SS-109 bullet and maybe they didn't even test 1:7 until they decided to due to the tracer rounds? While it says 1:9 is "ideal" it doesn't give any details about how 1:7 performs compared to 1:9, it simply says 1:7 was "settled" on. Does that mean 1:7 doesn't perform well or just as well?

What is the context/meaning of settle here?
At best this is inconclusive evidence
[/div]






Molons "Data" comes from 1 man firing 2 seperate weapons that have MANY variables that can influence the accuracy of the rounds fired. Some guy fring 30 rounds off the bench from 2 diffrent guns is NOT proof of anything, except on that day, he shot on better than the other.

If the data is not sourced from a barrel locked down in a test apparatus and laser bore sighted to the exact center of the test target for each round fired, with all the variables that could effect accuracy eliminated, then its NOT quantifiable data, no matter how many nice power point slides you make and post of the groups fired off the bench.

Also, you little trick with the dictionary was just, well, retarted.

And who the hell ever said anything about overstabilization? Optimal twist is far from over stabilization. I don't believe I have ever mentioned overstabilization even once. You need to go hash that out with someone else.
Molon  [Team Member]
10/5/2010 1:52:15 PM EST



In this article, we’ll be taking a look at the actual level of accuracy (technically precision) that is consistently achievable with the ubiquitous Wolf Performance Ammunition when fired from a chrome-lined, NATO chambered, semi-automatic AR-15. The “Performance Ammunition” evaluated for this article is loaded in polymer-coated steel cases and uses full metal jacket bullets with “bimetal” jackets; a three-layered affair consisting of a very thin outer layer of copper, a thick middle layer of steel and another thin layer of copper forming the inside of the jacket.



This accuracy evaluation used statistically significant shot-group sizes and every single shot in a fired group was included in the measurements. There was absolutely no use of any Group Reduction Techniques (e.g. fliers, target movement, Butterfly Shots). The shooting set-up will be described in detail below. As many of the significant variables as was practicable were controlled for. Also, "control groups" were fired from the barrels used in the evaluation using match-grade hand-loaded ammunition; in order to demonstrate the capability of each barrel. Pictures of the fired shot-groups will be posted for documentation.

All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) The barrels used in the evaluation were free-floated. The free-float handguards of the rifles rested in a Sinclair Windage Benchrest, while the stock of the rifles rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The set-up was very similar to that pictured below.






The Wind Probe.





The first Wolf Performance load evaluated was the 55 grain FMJ version. The test vehicle was a 20” Colt HBAR with a 1:7” twist, chrome-lining and a NATO chamber. A 10-shot control load was fired from this barrel at 100 yards using hand-loaded Sierra MatchKings. The extreme spread of that group was 1.03”.










Four 10-shot groups of the 55 grain FMJ Wolf ammunition were fired in a row from 100 yards with the resulting extreme spreads:

4.22”
3.73”
3.34”
4.31”

for a 10-shot group average extreme spread of 3.9”. All four of the 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius for the composite group was 1.16”.


The “smallest” 10-shot group.






The 40-shot composite group.





Knowing that the Internet Commando would feel that the above evaluation was unfair, due to his false belief that a 1:7” twist barrel “over-stabilizes” 55 grain bullets, I repeated the entire evaluation using a 1:9” twist barrel. The results were nearly identical.

The new test vehicle was a 16” Colt HBAR with chrome lining, a NATO chamber and of course the 1:9” twist. This is the barrel found on the Colt 6721 carbine. This barrel was also free-floated. The exact same lot of Wolf 55 grain FMJ ammunition was used for this repeated evaluation. All other aspects of the evaluation were exactly as described above for the previous evaluation.





A 10-shot control group fired from the 16” Colt HBAR from 100 yards using the same match-grade hand-load of Sierra 55 grain BlitzKings had an extreme spread of 0.89”.


Just as before, four 10-shot groups of the 55 grain FMJ Wolf ammunition were fired in a row from 100 yards. The extreme spread of those groups measured:

4.30”
3.62”
4.04”
3.99”

for a 10-shot group average extreme spread of 4.0”. As before, all four of the 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. This composite group had a mean radius of 1.21”.



The “smallest” group from the 1:9” twist barrel.






The 40-shot composite group from the 1:9” twist barrel.




Here are both of the 40-shot composite groups side by side for comparison.








As previously posted, the exact same conclusions were reached when performing the same test using Prvi M193 ammunition. Using the Prvi M193 ammunition, I did an accuracy comparison firing the 55 grain FMJ load from both a 1:9” twist Colt HBAR and a 1:7” twist Colt HBAR. Four 10-shot groups were fired from each barrel from a bench-rest at a distance of 100 yards. The groups from each barrel were over-layed to form 40-shot composite groups. The mean radii of the composite groups were nearly identical.




C_Rion  [Team Member]
10/5/2010 1:54:51 PM EST
Once again, rifles off the bench with many variables that can influence accuracy from shot to shot. Nice pictures and write up, but dosent change what it is.

LOL, wolf ammo? The most consistent ammo made for an accuracy test such as this. Im done here, the "data" is getting silly. I think Ill go shoot my 1/9 for a while and finish up with a 1/8. Have fun folks!
Quintanaroo  [Team Member]
10/5/2010 2:01:25 PM EST
Originally Posted By C_Rion:
What weight ammo are you going to shoot?

Heavy Match Grade ammo= 1/7

Standard off the shelf 55 and 62 grain ammo = 1/9

Now some history....

FN states that the optimal twist for the ss109/M855 service round is 1/9.
Colt stated during the A2 work up that the barrel twist should be in 1/9.
BOTH companies practically begged the govt to go with a 1/9 twist.
javascript:openHelp('/lib/util/boxAnims.html');
1/7 was adopted as a compromise because of an abnormally long, poorly designed US made tracer, the M856

1/7 Is very popular with the ARFCOM fanboy crowd, who imagines themselves fighting the apocalypse with 77 grain OTM

The companies that have the professional ballisticians, who actually made the ammunition and the a2 rifle say 1/9 is optimal for the current service ammo.

Most people shoot only off the shelf 55-62 grain ammo, and are better served with the 1/9 twist

Now you can make an informed decision


Interesting!! Thanks.

I am going to research this

Krylancelo  [Team Member]
10/5/2010 2:23:28 PM EST

Originally Posted By C_Rion:
Also, you little trick with the dictionary was just, well, retarted.

Sorry, that's just funny.
Krylancelo  [Team Member]
10/5/2010 2:25:10 PM EST

Originally Posted By C_Rion:
And who the hell ever said anything about overstabilization? Optimal twist is far from over stabilization. I don't believe I have ever mentioned overstabilization even once. You need to go hash that out with someone else.

Then feel free to explain to the class exactly what optimal means. How is 1:7 less-than-optimal? What differences will be experienced between 1:9 and 1:7 with the same ammunition? You keep on going on and on about how 1:9 is optimal. Okay. So what makes 1:7 sub optimal? How is it worse? What are your sources? What testing was done if any?
Krylancelo  [Team Member]
10/5/2010 2:27:13 PM EST

Originally Posted By C_Rion:
Once again, rifles off the bench with many variables that can influence accuracy from shot to shot. Nice pictures and write up, but dosent change what it is.

LOL, wolf ammo? The most consistent ammo made for an accuracy test such as this. Im done here, the "data" is getting silly. I think Ill go shoot my 1/9 for a while and finish up with a 1/8. Have fun folks!

So you're laughing at the guy who is providing information to back up his claim, even if it isn't completely 100% scientific and yet you won't provide any information at all? Do you even own a 1:7 barrel?
David14  [Team Member]
10/5/2010 2:46:54 PM EST
OP, I had a 1-in-9" heavy-barrelled 20" upper, which was plenty accurate, and a great gun for shooting prone. But it was simply too heavy for shooting standing & unsupported (with a sling it wasn't too bad), and I ended up selling it. If you're going to be doing a bunch of shooting off of a bench, then the heavy barrel won't make a difference. If you're going to be shooting offhand a bunch, or carrying the rifle any distance, I think you'll be happier with the gov't profile.
Just my 2 cents' worth.
Molon  [Team Member]
10/5/2010 3:39:22 PM EST
Quality 55 grain bullets can shoot superbly from fast twist barrels. Most 55 grain full metal jacket bullets do not fall into the quality category. The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist.





The 3-shot group pictured below was fired from the same 1:7” twist barrel for the Internet Commandos in our viewing audience.







The next 10-shot group pictured was fired from a Krieger barreled AR-15 with a 1:7.7” twist using 55 grain BlitzKings.




Barrels with 1:7" twists can shoot bullets as light in weight as 40 grains without issue, as long as it is a modern, quality 40 grain bullet, such as the Hornady 40 grain V-MAX.








Here's another example of just how well a 1:7" twist barrel can shoot light weight bullets. While the group pictured below was fired from a distance of only 50 yards, it's a 10-shot group fired from a chrome lined, NATO chambered Colt barrel with a 1:7" twist using 52 grain Sierra MatchKings.








A 14.5" Colt M4A1 SOCOM barrel,with its NATO chamber, chrome lining and 1:7" twist can shoot quality 55 grain bullets quite well.





borderpatrol  [Member]
10/5/2010 4:10:38 PM EST
I still own an A1 barreled upper (1/12) that shoots 52 grain SMK's very well despite having a very rough throat. It's actually difficult to get patches started just ahead of the chamber. It's shot out.

The 1/7 or 1/8 will guarantee anyone the ability to shoot all the available ammo for .223. The 1/9 twist barrels can't guarantee it. Some will and some won't. It's only after the money has been spent will the shooter find out.

The safe bet, buy a 1/7 and shoot any weight bullet you like without issues.

cbsaf  [Team Member]
10/5/2010 4:30:06 PM EST
Del-Ton makes a 20" 1/9 barrel that is also a GI A2 profile. I can't believe I would be the first to say.....get both.
MistWolf  [Member]
10/5/2010 6:20:17 PM EST
Originally Posted By Blain: At first I instantly thought the GI profile was the way to go. Slimmer profile means less weight and faster cooling...
Where do people come up with this stuff? Sure it cools off quicker, but it heats up faster. The barrel still has to deal with the same amount of heat but with less mass. Less mass means the steel will reach higher temperatures. Higher temperatures means more throat erosion. It doesn't mean your rifle will need a barrel replacement after a couple of magazines, but it does mean IN GENERAL the lighter barrel will wear out faster. The barrel is one place on a rifle where weight is good!

The HBar profile barrel isn't so heavy that you'll be limited to shooting off a bench. I had a Colt 20" HBar and I humped that over many miles while hunting, even with a big scope. It wasn't that bad, boys & girls. Mine was a bit muzzle heavy- just enough to hang nicely while shooting offhand. Still, it balanced nicely. Bull barrels on the other hand, are heavy.

Twist rate you'll have to work out for yourself. 1:7s will shoot the lighter bullets fine. 1:9 from a 20" barrel will shoot bullets up to 70 gr fine and likely all the way up to 77 gr. It's not the twist rate that stabilizes bullets, people. It's RPM. Since a 20" barrel generally spits out the bullets at higher velocities, they will exit at a higher RPM than a shorter barrel with the same twist. The only way to know if a barrel will shoot a certain bullet well is to shoot it.

A 1:7 will pretty much guarantee that it will stabilize any bullet up to 80 gr but at a price. There will be an increase in barrel wear and a slight loss of velocity.

You cannot over stabilize a bullet. It's either stable or it's not. If the bullet's RPMs are too low to stabilize it, it's unstable. If they are too high and destabilize the bullet, it's unstable. Bullets are more stable at some RPMs than they are at others.

Everything is a compromise, whether it's barrel weight, twist, length or what have you. The laws of physics do not change just because we are talking about ARs
CJan_NH  [Team Member]
10/5/2010 7:52:17 PM EST
Do you even own a 1:7 barrel?

I'd venture to opine that most people who bash 1:7 barrels don't-otherwise they would realize how silly their argument is.

The rest of us are mere "ARFCOM fanboys" () That retort is the typical tired blather from the intellectually vacant keyboard commando who doesn't have anything else to bring to the argument, beside his opinion.

The three keywords that the 1:7 bashers always latch onto are "less than optimal". Okay, fine...what does "less than optimal" mean in terms of performance and longevity?

It means precisely zilch.

I've got a safe filled with 1:7 and 1:9 barreled rifles, with a couple of 1:8 and 1:12 rigs thrown in for flavor. There is nothing that my 1:9 rifles can do that my 1:7 or 1:8 rifles can't. On the other hand, one of my 1:9 barreled Rock River ARs can't shoot 75gr ammo worth a damn, even though the rest of my 1:9 barreled rifles can handle it just fine.

Go figure...
MTNmyMag  [Team Member]
10/5/2010 8:02:11 PM EST
1-7 will pretty much let you run eveything. I dont have a 1-7 just 1-9s and 1-8. Wont make this mistake in the future.
Bretshooter  [Member]
10/5/2010 8:04:07 PM EST
I am with you, but... I amended your statement a bit.

Originally Posted By Krylancelo:
So you're laughing at the guy who is providing information to back up his claim, even if it isn't completely 100% scientific your preferred brand of ammo and yet you won't provide any information at all? Do you even own a 1:7 barrel?

C_Rion  [Team Member]
10/5/2010 11:09:10 PM EST
Originally Posted By CJan_NH:
Do you even own a 1:7 barrel?

I'd venture to opine that most people who bash 1:7 barrels don't-otherwise they would realize how silly their argument is.

The rest of us are mere "ARFCOM fanboys" () That retort is the typical tired blather from the intellectually vacant keyboard commando who doesn't have anything else to bring to the argument, beside his opinion.

The three keywords that the 1:7 bashers always latch onto are "less than optimal". Okay, fine...what does "less than optimal" mean in terms of performance and longevity?

It means precisely zilch.

I've got a safe filled with 1:7 and 1:9 barreled rifles, with a couple of 1:8 and 1:12 rigs thrown in for flavor. There is nothing that my 1:9 rifles can do that my 1:7 or 1:8 rifles can't. On the other hand, one of my 1:9 barreled Rock River ARs can't shoot 75gr ammo worth a damn, even though the rest of my 1:9 barreled rifles can handle it just fine.

Go figure...


Whatever CJan. Ill bet my whole retirement check that I made it a bit farther than "keyboard commando" and way farther than yourself. What branch did you get your "commando" certification in? ARFCOM? Figures. I swear, I just cant understand why so many people have emotional investments in the Specs of their rifles. Say something contrary, and they respond like you directly attacked them, instead of the twist of a piece of steel on a damn rifle.

To the 1/7 barrel question, yes. Both of my LMT carbines are 1/7 twist. They are also loaded with 77 Gr SMK, and since these are house guns, they have the optimal twist for the ammunition loaded. However, For my first love, 20 in A2/A4 rifles, I shoot alot, and its all off the shelf 55-62 grain ammo. Therefore, I have those rifles barreled in 1/9 twist, which is the optimal twist for 55-62 grain ammo. There are other minor benefits for a 1/9 shooting only 55-62 grain ammo, less velocity loss, longer barrel life. The OP asked about a 20 inch gun, and I would surmise that he is probably going to only shoot off the shelf 55-62 grain ammo. In this case, I stand by my first assesment, 1/9 is the better twist for his application.


Once again, im done with this thread. I have said what I wanted to, and thats all that really matters.
Blain  [Member]
10/6/2010 2:55:54 AM EST
A 20" 1-7 twist will not have as much velocity as a 20" 1-9 twist barrel? Wow, I never heard that before. Just how much velocity loss are we talking here?
Molon  [Team Member]
10/6/2010 4:51:10 AM EST
Originally Posted By Forest:

In God we trust, all others must bring data.



Now where have I heard that before?

Molon  [Team Member]
10/6/2010 5:00:16 AM EST
According to C.E. Harris, testing at Aberdeen Proving Ground has shown that “For military use you generally want Sg (the gyroscopic stability factor) to be in the range of 1.5 - 2.0 . . .”

For the M855 bullet, a gyroscopic stability factor of approximately 1.5 (depending upon velocity) will be obtained with a twist rate of 1:8.3”.







For the M855 bullet, a gyroscopic stability factor of approximately 2.0 (depending upon velocity) will be obtained with a twist rate of 1:7.2”.






Additionaly C.E. Harris reported that testing has shown that “bullets which are well balanced, more than adequate twist does not hurt until you get to a gyroscopic stability factor greater than 5.0.” To put that into perspective, a Hornady 55 grain FMJ bullet would have to be fired from a barrel with a 1:6” twist rate at a muzzle velocity of 4000 fps to obtain a gyroscopic stability factor greater than 5.0.
HeavyMetal  [Team Member]
10/6/2010 5:31:07 AM EST
Originally Posted By C_Rion:
Originally Posted By CJan_NH:
Do you even own a 1:7 barrel?

I'd venture to opine that most people who bash 1:7 barrels don't-otherwise they would realize how silly their argument is.

The rest of us are mere "ARFCOM fanboys" () That retort is the typical tired blather from the intellectually vacant keyboard commando who doesn't have anything else to bring to the argument, beside his opinion.

The three keywords that the 1:7 bashers always latch onto are "less than optimal". Okay, fine...what does "less than optimal" mean in terms of performance and longevity?

It means precisely zilch.

I've got a safe filled with 1:7 and 1:9 barreled rifles, with a couple of 1:8 and 1:12 rigs thrown in for flavor. There is nothing that my 1:9 rifles can do that my 1:7 or 1:8 rifles can't. On the other hand, one of my 1:9 barreled Rock River ARs can't shoot 75gr ammo worth a damn, even though the rest of my 1:9 barreled rifles can handle it just fine.

Go figure...


Whatever CJan. Ill bet my whole retirement check that I made it a bit farther than "keyboard commando" and way farther than yourself. What branch did you get your "commando" certification in? ARFCOM? Figures. I swear, I just cant understand why so many people have emotional investments in the Specs of their rifles. Say something contrary, and they respond like you directly attacked them, instead of the twist of a piece of steel on a damn rifle.

To the 1/7 barrel question, yes. Both of my LMT carbines are 1/7 twist. They are also loaded with 77 Gr SMK, and since these are house guns, they have the optimal twist for the ammunition loaded. However, For my first love, 20 in A2/A4 rifles, I shoot alot, and its all off the shelf 55-62 grain ammo. Therefore, I have those rifles barreled in 1/9 twist, which is the optimal twist for 55-62 grain ammo. There are other minor benefits for a 1/9 shooting only 55-62 grain ammo, less velocity loss, longer barrel life. The OP asked about a 20 inch gun, and I would surmise that he is probably going to only shoot off the shelf 55-62 grain ammo. In this case, I stand by my first assesment, 1/9 is the better twist for his application.


Once again, im done with this thread. I have said what I wanted to, and thats all that really matters.



Microscopically better perhaps. Most of the 1/9 twist barrels are 4140 too so I doubt you will be getting a longer lifespan than a mil-spec 1/7.

Why preclude your ability to shoot 75-77gr stuff in favor of such a trifling optimization for these lighter bullets?

As Molon has so ably demonstrated, the difference accuracy-wise is incosequencial.
Krylancelo  [Team Member]
10/6/2010 6:00:41 AM EST

Originally Posted By Blain:
A 20" 1-7 twist will not have as much velocity as a 20" 1-9 twist barrel? Wow, I never heard that before. Just how much velocity loss are we talking here?

I honestly believe this is myth.
Rakin  [Member]
10/6/2010 7:10:50 AM EST
Thank you Molon for supporting this thread with your concise write ups. I appreciate the time and investment you place in the forum (and avoidance of the thread noise)


Rakin
Forest  [Moderator]
10/6/2010 9:55:58 AM EST
Originally Posted By Molon:
Originally Posted By Forest:

In God we trust, all others must bring data.



Now where have I heard that before?



QCMGR's sig line It's been adopted as the unoffical Tech Forums motto.
MistWolf  [Member]
10/6/2010 5:53:49 PM EST
Originally Posted By Krylancelo:

Originally Posted By Blain:
A 20" 1-7 twist will not have as much velocity as a 20" 1-9 twist barrel? Wow, I never heard that before. Just how much velocity loss are we talking here?

I honestly believe this is myth.
It takes more energy to drive a bullet through tighter rifling. It's not a myth but it's not enough to worry about either. When you start comparing data, you'll see a trend.

It's wrong to assume that the 1:9 will not stabilize bullets heaver than 69 gr especially when the barrel is 20". Of course, there is a slim chance it might not, just like a 1:7 twist barrel may not shoot the lighter bullets well. But you won't know until you shoot it. Sometimes ya pays yer moneys and takes yer chances

About that stability chart- If the M855 uses a 55 grain bullet, I'd take that chart with a grain of salt. Many folks have good accuracy with 55 gr bullets and twists as slow as 1:12. If the M855 uses a 62 grain bullet, the same. Many folks have good results with 62 gr bullets & the 1:9

FRCA_WY  [Member]
10/7/2010 6:53:30 AM EST
It's been my experience that 1X9 will shoot 55FMJ out of the box ammo better than 1X7.

Now,before anyone throws a fit,this is MY experience with my two 1X9 rifle/carbine,and two 1X7 rifle /carbine. As well as a few others that don't belong to me....................YMMV,etc,etc.

If that is what you normally shoot,and that's true for me,then take heed, if not,then 1X7 has its advantages.

Get both!
Blain  [Member]
10/7/2010 4:49:07 PM EST
One thing I just thought of that I totally neglected to think of before......forget a .22 conversion if you have a 1-7 twist barrel....
JAD  [Member]
10/7/2010 5:55:54 PM EST
The 1/9 may or may not stabilize the 77 gr stuff in the colder weather. Plus, unless you are going to spend all your time shooting this off of a bench, a full length H-Bar just plain sucks for dead weight to carry.

That's more or less a personal thing. I have carried a 20" HBAR in the woods and it is not a problem at all. I can tell the difference between it and a pencil barrel (I don't have a government profile to compare against, though I have had the opportunity to carry one and I don't recall much difference between the feel of it and the HBAR), but it isn't at all uncomfortable for me to carry an HBAR. It may be for some people...

The heavier rifle can also be more forgiving on field position shots.
Krylancelo  [Team Member]
10/8/2010 7:35:41 AM EST

Originally Posted By Blain:
One thing I just thought of that I totally neglected to think of before......forget a .22 conversion if you have a 1-7 twist barrel....

Just my opinion but if you want to do .22, buy a dedicated upper. The conversions seem hit or miss.
Blain  [Member]
10/9/2010 3:12:38 AM EST
If you are on patrol, it's easier to carry a conversion kit, than an extra upper.
53vortec  [Team Member]
11/29/2010 5:05:39 PM EST
Just a related anecdote I thought of when reading this thread that I turned up in a search.

A few years ago I was having some problems with a barrel made by Sabre Defence. For those unfamiliar, they have been a major barrel maker for the US military for some years. While talking with them I mentioned I'd tested the 1/7 twist barrel with 55gr ammunition - while they didn't attribute the problems to using that combo, they did suggest I might want to go to a 1/9 twist if I was just going to be shooting 55gr bullets. I told them I typically shoot 77gr, we didn't discuss the matter further. In hindsight I wish I'd have had them elaborate on their reasoning.

(Before I get dogpiled, let me make it clear I'm not saying a 1/9 is better for 1/7, or even that that is Sabre's official standpoint.)

Regarding over-stabilization that many are quick to talk about, the phenomenon has been observed - in ARTILLERY. In rifled indirect fire weapons, too fast a rate of twist can result in the projectile landing base first.
bcw107  [Team Member]
11/29/2010 5:28:09 PM EST
Tag

Posted Via AR15.Com Mobile
Paid Advertisement
--