55 Grain Bullets Fired From AR-15s with 1:7" Twist Barrels

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"55 grain bullets are unstable/overstabilized/inaccurate/less-lethal when fired from an AR-15 with a 1:7” twist barrel."

The Internet Commando

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Statements such as the one above always seemed to be proclaimed by the Internet Commando, without posting any valid data to support that these effects occur, or that if they do occur, they do so to any degree that has any significant effect on the terminal ballistic properties or accuracy/precision spectrum involved with AR-15s firing M193 type ammunition.


By definition, an “unstable” bullet will have a gyroscopic stability factor of less than 1.0 at the muzzle.  A typical 55 grain FMJ bullet will have a gyroscopic stability factor of approximately 4.27 when fired from a 20” barrel with a 1:7” twist.  [CaptainObvious] 4.27 is not less than 1.0. [/CaptainObvious]


The following demonstration compares the results of firing four 10-shot groups of the same lot of 55 grain Prvi Partizan M193 ammunition from two different barrels; one barrel with a 1:9” twist, the other barrel with a 1:7” twist.

The first barrel used in testing was 16” Colt HBAR with chrome-lining, a NATO chamber and a 1:9” twist.  This is the barrel found on the Colt 6721.  All of my free-floated Colt 6721 barrels have turned in sub-MOA 10-shot groups at 100 yards when using match-grade hand-loads.

The second barrel used in testing was a 20” Colt HBAR, also with chrome-lining, a NATO chamber and of course a 1:7” twist.  I've owned three of these barrels and they have all turned in 10-shot groups at 100 yards that hover just above one MOA when free-floated and shooting match grade handloads.  The longer barrel with the 1:7” twist was purposely chosen for the increased muzzle velocity coupled with the 1:7” twist.  

Accuracy (technically, precision) testing was conducted from a distance of 100 yards following my usual protocol. The barrels were free-floated during testing.  The fore-ends of the weapons rested in a Sinclair Windage Benchrest and the butt-stock rode in a Protektor 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. Naturally, the wind conditions were monitored using a Wind Probe. The set-up was very similar to that pictured below.









Four 10-shot groups of the PPU M193 were fired from the 1:9” twist barrel. Those groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius of that composite group was 1.08”.

As with the 1:9” twist, four 10-shot groups were fired from the 1:7” twist barrel.  Those groups were also also over-layed on each other to form a 40-shot composite group; the results were nearly identical to those obtained from the 1:9” twist barrel.   The composite group had a mean radius of 1.01”. The two composite groups are shown side by side for comparison.









The entire test as described above was also conducted using a second 16" chrome-lined, NATO chambered Colt HBAR with a 1:9" twist and a second 20" chrome-lined, NATO chambered Colt HBAR with a 1:7" twist.  The ammunition used in this test was all from the same lot of Wolf 55 grain FMJ "Performance Ammunition."  

As before, four 10-shot groups fired from each barrel at 100 yards were over-layed on each other using RSI Shooting and just as before, the mean radii for these 40-shot composite groups were nearly identical.









Testing performed by C.E. Harris at Aberdeen Proving Ground and later at Sturm-Ruger has shown that the “overstabilization” claim is largely nonsense. The testing showed that “overspinning” quality light-weight bullets from a fast twist barrel does not become an issue unless you have a gyroscopic stability factor greater than 5.0 (which would require something along the lines of a 1:6” twist barrel launching a 55 grain bullet at over 3500 fps) or unless firing at an angle greater than 85 degrees.

After the U.S. Military adopted the 1:7” twist for the M16A2, C.E. Harris did extensive testing comparing the accuracy of light-weight bullets fired from 1:10” twist barrels and 1:7” twist barrels using 52 grain Sierra MatchKings. The accuracy testing was done from 200 yards, (well into the downward slope of the trajectory where the Internet Commando claims that all manner of evil befalls the “overstabilzied” bullet), and the accuracy results from the two different twist barrels were also nearly identical.



Quality, modern lightweight bullets of  copper-jacket/lead-core construction can shoot superbly from AR-15s with fast twist barrels.   Typical 55 grain FMJ 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 using 55 grain BlitzKings.










For any Internet Commando’s in our viewing audience, the 3-shot group pictured below was fired from the same 1:7” twist barrel using 55 grain BlitzKings.









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









The group pictured below was fired from one of my Krieger barreled AR-15s with a 1:7.7" twist from a distance of 100 yards using Berger 55 grain HP bullets.  The group has an extreme spread of 0.76".











The next group was fired from my 1:7.7" twist Krieger barreled AR-15 using factory loaded Hornady American Gunner 55 grain HP ammunition.










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.










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 1:7”  twist rate does not have a negative effect on the terminal ballistic properties of M193 and M855.  

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."



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Thanks for posting this.

Good read... Nice info.

"Quality,
modern lightweight bullets of  copper-jacket/lead-core construction can
shoot superbly from AR-15s with fast twist barrels.   Typical 55 grain
FMJ bullets do not fall into the quality category."





Spot on!





Good to have you back Molon and a great post as usual.



 

Thanks for posting. I remember reading an American Rifleman article that mentioned how 1/9 was more accurate than 1/7 with 55 gr and it never seemed right to me. The author made several unsubstantiated claims about 1/9 twist being more accurate.



You test provides data that this is incorrect. The author didn't provide sources for his claims or even pictures of an accuracy test.

I know you'll appreciate this gem as well:


http://www.americanrifleman.org/articles/2014/5/21/testing-the-army-s-m855a1-standard-ball-cartridge/

As always, a pleasure to read your post. Thank you for sharing the fruit of your time and energy.

Great data Molon.

You should check out the GM barrel test thread. We tested various fast twist rates, up to 1/5. What was very interesting was that with the 1/6 twist barrels, the 55gr pills seemed to fly the best out of all the weights tested (45gr up through 80gr).

An Accuracy Comparison of M855 When Fired From AR-5s With 1:9” Twist and 1:7” Twist Barrels.






This test will compare the accuracy (technically, precision) of M855, produced by Israel Military Industries (IMI), when fired from an AR-15 with a chrome-lined, NATO chambered barrel with a 1:9” twist and an AR-15 with a chrome-lined, NATO chambered barrel with a 1:7” twist.  The IMI M855 is the singularly most accurate and consistent loading of all the M855 clones that I have tested to date.

IMI M855 is loaded with a 62 grain FMJ bullet with a cannelure. It also has a steel penetrator seated in the ogive portion of the projectile. This makes the projectile unusually long for its weight, as well as giving it a lower specific gravity. The ammunition is charged with "ball powder". The primers are sealed and crimped in placed. The bullet is also crimped in place and the case mouth is sealed with an asphalt sealant.









Chronograph data for the IMI M855 was obtained using an Oehler 35P with "proof screen" technology. All velocities listed are muzzle velocities as calculated from instrumental velocities using Oehler's Ballistic Explorer software program. All strings of fire consisted of 10 shots each.









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 the barrels.

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 handguard of the rifle rested in a Sinclair Windage Benchrest with the aid of a Sinclair fore-arm adaptor, while the stock of the rifle 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. All rounds were fired from the magazine. Each upper was fired using the same lower. The set-up was very similar to that pictured below.









The barrels used in this evaluation were both Colt HBARs with NATO chambers and chrome lining. The 1:7” twist barrel was 20” in length and the 1:9” twist barrel was 16” in length. I purposely selected the shorter barrel with a 1:9" twist and the longer barrel with the 1:7" twist in order to exacerbate any possible statistically significant influence that the differing twist rates might have on the precision of the M855 ammunition.

The first barrel used in testing was 16” Colt HBAR with chrome-lining, a NATO chamber and a 1:9” twist. This is the barrel found on the Colt 6721. All of my free-floated Colt 6721 barrels have turned in sub-MOA 10-shot groups at 100 yards when using match-grade hand-loads.

The second barrel used in testing was a 20” Colt HBAR, also with chrome-lining, a NATO chamber and of course a 1:7” twist. I've owned three of these barrels and they have all turned in 10-shot groups at 100 yards that hover just above one MOA when free-floated and shooting match grade handloads.



Colt 16" HBAR





Colt 20" HBAR







Prior to the firing of the M855, control groups were fired from the test barrels using match-grade handloads seated with Sierra 55 grain BlitzKings. The 10-shot control group fired from the 1:7” twist barrel had an extreme spread of 1.17” and the 10-shot control group fired from the 1:9” twist barrel had an extreme spread of 0.89”.


Three 10-shot groups were fired from the upper with the 1:9” twist barrel in the manner described above. The extreme spreads for those groups measured:

2.72”
2.19”
2.24”

for a 10-shot group average of 2.38”. The three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for the composite group was 0.76”.



In the same manner as above, three 10-shot groups were fired from the upper with the 1:7” twist barrel. The extreme spreads of those groups measured:

2.14”
3.01”
1.71”

for a 10-shot group average of 2.29”. As before, the three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for this composite group was 0.68”.




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excellent information



thank you for posting it



two thumbs up

Awesome read.  Thanks for posting.

Nice report! Thanks for posting

I've found the same over the past 10 years, 1:7 or 1:10 wouldn't make a bit of difference with 55 gr. bullets.

55 gr Sierra Blitzkings is the King of 55 gr bullets for sure!

I will have to try some Blitzkings now..thank you for the great post and that's some AMAZING shooting skills.

Threads like these are what really make this site.

Good info

Ive been shooting 55 gr out of a 1:7 twist for years with no issues

Thank you, your work is one of the reasons I pay for a membership.  

I'm glad to read your evaluation of IMI M855, I just bought some of it. As always, excellent information.

Gotta love those 52 grain MatchKings.

I just want to see if there is a difference in gel at 100 yards with M193 fired from a 5.56 chambered 20" 1/7 twist vs say a 5.56 20" 1/12 or 1/14 twist. I have yet to see anyone do any decent test of this.

 
Agreed.  Thanks Molon, you give us some of the best real world information in a very well presented manner.  Great thread!

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."


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Very nice post, Molon.  I thought the controversy on this died a decade ago.

Nice write up

Both of my AR's are 1/7 and 55gr is the only weight that has ever been shot from them

Sadly, no.

Even 1/6 twist barrels can shoot 55s well:

I'll take it.

Nice!


That ADI 55 grain BK is a decent factory load for accuracy.






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Thanks!

Bad information, mis-information and mistakes on the internet are like gophers, just when you think you got them under control, another one pokes his head up....

Thank you very much for these posts Molon.

My mind is blow - I'll need some time to read this and digest it more.  For a very long time, I have thought this was false.

Put the lie to your name and start reading these posts to learn more.

Why 1/8 wylde then?
I understand the chambering, but why the twist rate?
And not off brand barrel makes either.

This all begs the question then how does each round do at 300 yards?  Do the results change much?  Thanks.

For most non-custom bullets, if it can be loaded to magazine length, it can be stabilized by a 1:8.
1:7.7 is better for the ~80/82gr Bergers and the like.
1:7 for tracers and tangent ogive 90gr, 1:6.5 for secant ogive 90gr, is my understanding. ETA: Or perhaps it could depend on altitude and such.
 

 
And, 1:5 for exploding Barnes 36gr Varmint Grenades and killing chronographs

Lulz
Turned into snake shot huh?

1/8 is used in semi auto weapons that are magazine fed. So is this the venn diagram that brings the Varmint grenades ie 35-55gr into play?

Is it possible for you to compare M856 in a 1:9 and 1:7 barrel?




 

That would be interesting.  Unfortunately, the DNR ranges that I do most of my precision testing at don't allow the use of tracers.

After reading this more closely, I have to say a big Thanks!  This is some of the best work on the topic ever posted.   Strong evidence that the 1/9 advantage over 1/7 is overstated.  Differing from what I had been holding as my own opinion.  It would be interesting to see how a low-quality but high-velocity bullet (at the same time), does.  Something like Federals 5.56 line, and in particular, their 62 gr greentip 5.56.


One component of the claim that 1/9 is better than 1/7 (but not really substantiated by the proponents), is that it is better with cheap (out of balance) bullets (as per the NRA article: though alas, that would be the one which offered no data or references).  In theory, the claim is the high RPM's causing the poorly balanced bullet to warble.  The claim seemed credible enough, and I've talked to a respected (state champion level) high power shooter who claimed to be able to watch a bad batch of poor quality of-spec bullets spiral, on the way to the 600 yard line.  (At those ranges, you can watch the contrails in a spotting scope, with the right light).  So it seemed a credible claim that an out of balance bullet can warble, and harm accuracy - and that excessive RPM's can make that worse.

To address that, Molon's excellent Wolf and IMI-SS109 tests are the most significant to me - thank you!  Unfortunately, While Wolf is a known low-quality bullet, it's also low velocity (lower RPM's).   IMI is 62 gr M855 at full velocity, but IMI is generally considered the best quality most accurate 62 gr (i.e. least out-of-balance bullet).  

I would be particularly interested if test work of this quality could be done with Federal LC brass'ed XM193 5.56, and Federal LC brass'ed 5.56 62 gr green tip -which would be the low quality bullet at high velocity test, that would really put the nail in coffin on this.  of note, it seems like most tests, such as the Aberdeen tests, all seem to go out of the way to pointing out that twist rate affect on accuracy it is irrelevant - with quality bullets.  That qualifier at the end is the item of note, why is that always added?    Obviously sir, you aren't obligated to do a damned thing!  


As to the question on why do match rifles have 1/8 twist?  I think that's a good one.  It was once said (But never substantiated), that 1/7 burns out faster, and I always guessed 1/8 was chosen because SS match barrels do wear out much faster than regular barrels, and most match shooters will replace in as few as 3000 rounds.  So perhaps 1/8 was chosen for a longer BBL life than a 1/7 match SS BBL would?  I can say from personal experience, the accuracy effect of a worn stainless steel match BBL after 4000 rounds is not something you will see at 200 yards or even 300 yard rapids, and generally only shows up on the 600 yard line.  I'm curious if Molon has an opinion on twist rate affect on BBL life.  For most chrome lined regular steel BBLs used by civilians in semi-auto fire, I imagine it's nearly irrelevant.

Darn, I thought I had a freak 1:7 14.5 bbl that shoots 1 moa 55gr geco!  Thanks for the research molon

excellent post  

I get so tired of the retarded anti-1:7 twist propaganda

*****moderators need to make this thread a sticky*****

There is no substantial difference in the muzzle velocity of Federal XM193 compared to the PPU M193 used in this test, nor was there any significant difference in the accuracy of those two loads when fired from the same barrel.













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Again, there isn’t a substantial difference in the muzzle velocity of the 55 grain Wolf ammunition (which is a 223 Remington load) used in this test compared to American Eagle 223 Remington 55 grain FMJ












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as usual, Molon is the man.



Have been using M193 out of 1/7 and 1/8 bbls with great success.



Thanks

Thank you.  With that data, I agree that the velocity difference between XM193 and the PPU is probably negligible.  Also, I cross referenced performance between PPU and XM193, just to see if the PPU bullet is more accurate, which could skew the test conclusions.   From your M193 Review.  Indeed, there was nothing to suggest that the PPU bullet is any better quality than XM193, and in fact it performed slightly worse in that separate comparison.  The point of this paragraph is that cross referencing that data for possible holes, I find it is all consistent with your conclusions on the 1/7 vs 1/9 debate.  

I appreciate this post - though have to admit, it's annoying that apparently I've been wrong for a long time.