Does the rate of twist help determine how well the rifle can stabilize a certain weight bullet? What are the pros/cons of 1:7, 1:8, & 1:9 twist rates? The faster the twist, the heavier bullet it can stabilize? For 5.56, what are general recommendations of bullet weights for the 3 twist rates?

Rule of thumb:  the faster the twist rate the heavier the bullet

Longest (heaviest) that will stabilize in most conditions. The faster the twist, the more likely that ball ammo will group worse.
1:9 - 75gr HPBT
1:8 - 82gr LRBT
1:7 - 90gr VLD
http://www.bergerbullets.com/twist-rate-calculator/





IMO, the fastest needed for any mag-length ammo in any condition is 1:8.





 

Not all 1/9 twist barrels will stabilize 75 grain bullets. This is especially true with shorter barrels and longer ranges. To be 100% sure your rifle will stabilize 75 grain and heavier bullets you need a 1/8 twist.

1/9 is mathematically ideal for 68/69 grain match bullets. I have a 26" barrel 1/9 twist Winchester M70 that hammers with 75 grain Hornady hpbt's at 200 yards. About 50% of the people who own 1/9 twist barrels report they can't shoot 75 grain Hornady bullets without keyholing (sideways) through the target with horrible accuracy.  

There are manufacturing tolerances when making barrels and the end result is some 1/9's are really 1/9.25" and others are 1/8.75" Depending on where your rifle barrel falls within this range will determine whether it works with heavy bullets or not. You can buy a barrel that you know will work (1/8 or 1/7) or you can buy a 1/9 and try it. No guarantee comes with a 1/9.

1/8 and 1/7 twist barrels shoot light bullets very accurately. 52/53 grain match bullets are usually the most accurate at 100 yards from any of my barrels regardless of twist rate.

My Colt 6721 groups 62gr tighter than a gnat's ass; M855 is the only thing I use for this shooter...

Sarcasm, correct?



 

No sir; the 6721's HB is one of the more accurate "off the shelf" barrels out there.

I shot 55, 62 military 5.56 ammo, and hand loaded Sierra Match Kings in 77, 80 (their VLD) and 90 grainers through my 1 in 9 twist 16 inch carbine length gun.

Everything shot fine and accurate through the 77 grainers (loaded to magazine length).  The 80 and 90 grain SMKs keyholed like crazy and accuracy was three times what the rest shot (1.5-2.0 MOA with an EOTech XPS) at 100 yards.

My old Colt Match HBAR, which I shot for CMP matches was a 20 inch heavy barrel, 1 in 7 twist.  It loved the 80 grain SMKs and shot the 90 grainers well, but I never shot many of them because the 80 grainers shot well enough.

Actually some M855 has become quite good.  Recent lots of Lake City (LC 13) are consistently printing at about 1.2" at 100 yards from my 1:9 nitride carbine barrel.  This is a considerable improvement in accuracy from the past, and is no fluke.

Picture for proof.  This was one of the first groups I shot from that lot.

Not to be argumentative here, but while there are theoretical differences between the various available AR barrel rifling standards, practically, they are almost totally irrelevant.  If you're using specialized bullets, you may need a special barrel with a special rifling rate.  Otherwise, there are only really "edge condition" situations to worry about: the very lightest bullets you're going to use, and the very heaviest ones, and the velocities you're going to be pushing those bullets to.

The only "problem" with using a faster twist than needed for some bullets is that a VERY few, very specialized bullets cannot handle the forces applied when you load them to stupidly high velocities.  Some "varmint" bullets that are MADE TO DISINTEGRATE should not be loaded to extremely high velocities at all, but especially if you're going to fire them through a fast-twist barrel.  Sure, in theory you could push one of those 40 grain "varmint grenade" bullets at well over 3300FPS, but since they're made to come apart, they will do that out of a too-fast rifled barrel.  If you're looking at 55gr FMJs as the lightest bullet you'll be shooting, then a 1:7 twist is fine for that and EVERY bullet heavier/longer than that.

I would also submit that if you see a difference in bullet stabilization (measured by bench rested accuracy) between a 1:9.25 and a 1:8.75 twist barrel, you're a superhuman marksman.  The tolerances involved in all the other components of a round are such that the difference between those two barrels is pretty much overwhelmed by everything else.

Finally, there is almost no connection between barrel length and bullet stabilization at even extreme velocities, either extremely low or extremely high.  If you're talking about a 4" barrel and expecting to see a problem with stabilization with a 1:10 twist rate, I recommend you pay attention to the standard rifling rates for 9mm pistols...the two most common rates are 1:10 and 1:16.

The accuracy problems with M855 have to do with the complexity of the bullet's construction.  Originally, the steel insert was basically just a profiled slug of steel to keep the nose of the bullet from deforming on impact with a hard target.  Some of those inserts were probably pretty inconsistent, and forming the bullet with the jacket, insert and lead core was a lot of steps to get right.

Today, many NATO countries' bullets are made with harder inserts, which helps maintain the bullet profile better on impact, but it also allows the insert to be made more precisely.  If the insert is more precise, the soft lead core will form more consistently around it and make the bullet more symmetrical and thus more inherently accurate - or probably "less inherently inaccurate."

I'm not suggesting that I (or anyone else) can see an accuracy difference between a 1/9.25 twist compared to a 1/8.75 twist outside of an individual barrel's traits. What people discover is one so called 1/9 twist will shoot 75/77 grain bullets just fine, the other 1/9 twist will keyhole. Too many people have reported keyholing 75/77 grain ammo from 1/9 twist barrels to ignore he fact that this twist rate is on the ragged edge for this weight/length bullet.

Higher velocities can overcome borderline twist rates. A 26" barrel with a 1/9 twist rate is more likely to stabilize a 75/77 than a 14.5 or 16" 1/9 twist will. It may not make sense on the surface of it, but the higher speed (+250 or more fps) actually reduces the need for as much twist. As velocity drops over distance the faster twists rates are required to keep the bullet stable.  

Virtually all long range rifles use twist rates faster than needed because the bullet will lose 1400+ fps velocity prior to engaging the target. The twist rate choice should be based on a 175 grain or 190 grain .30 bullet traveling at 1300 fps, not 2600+. Both of these bullets will stabilize in a 1/12, the 175 will stabilize in a 1/13 and shoot great at 600 yards or less. Both will do better at 1000 yards with a 1/10 because the lower velocity needs more spin.

It is always wiser to opt for more twist than just enough twist. The only time I wouldn't suggest this is when barreling for a true 100/200 yard bench rifle (6mmPPC etc.) intended for use with custom light weight flat base match bullets.  

There is no harm in buying a 1/8 or 1/7 provided you don't intend to run Hornady SX or Varmint Grenade style bullets at full speed. Those bullets need a slow twist rate. Most people would like to know there AR-15 barrel will shoot fine with the current m262 or equivalent ammo. A 1/9 is a crap shoot in that regard, that's why I don't recommend it.

I have three 1/9 barrels that are very accurate, two AR's and a Winchester M70. I use the AR's for combat style courses and let em rip. One year I took 1st place with my Win M70 in every 600 yard reduced course match fired at my gun club. I averaged 199 over six tournaments, shooting two 200's using 75 grain Hornady hpbt bullets over 23.5 grains of VihtaVuori N540. This bolt action shot 75's great, the 16" and 20"  AR not at all. The AR's hammer with 69 grain Sierras. There is absolutely nothing wrong with 1/9 for a general purpose rifle or when using slightly lighter (69 grain) bullets if it doesn't shoot the 75's.

No argument here.  All barrels are finicky in one way or another, and I believe that the 1:9 twist is just odd enough that not every barrel maker "gets it right."  Maybe the twist isn't exactly 1:9, or maybe the rifling just isn't particularly consistent, but I see more complaints that 1:9s aren't consistent overall than any other AR barrel twist.  Which is pretty much to my earlier point: 1:7 will work for 95+% of all .224" bullets, and the ones it isn't really suited for are pretty specialized.

I think the whole 1:9 and 1:8 thing is a solution in search of a problem.  Unless you're shooting ultralight varmint bullets, the whole premise behind the introduction of 1:9 and 1:8 rifling, that 1:7 won't stabilize "light" bullets, is bogus.  The Army didn't go to a rifling twist of 1:7 in order to (at the time) make much of their ammunition stockpile not shoot well.  They did to allow for MORE bullet weights/lengths to be effectively stabilized.  It wasn't even the M855 bullet that required the faster twist; it was the M856 tracer bullet that needed it.  The M856 bullet is very, very long, even though it's almost the same weight as the M855's bullet.

That's really good for ball ammo, but I wouldn't call that gnat's ass.



 

That's untrue.  Precision rifle shooter tend to favor the slowest twist rate that stabilizes whatever they need to shoot, this minimizes adverse effects.

Rotational velocity of projectiles does not significantly decrease as they go downrange, there is not a loss of stability.

That's some big-azz gnats.

1-9 twist should handle sub 50gr through 69gr and some 75s without issue.  In shorter barrels you may want more twist for slower muzzle velocities although it's correct that spin doesn't slow down much after the bullet leaves the barrel (not like velocities do).  FYI it's not bullet weight, but length that matters, it's just that most heavy bullets are also long.

1-8 and 1-7 should shoot the entire range of common AR loads without issue.  My 1-8 twist shoots 75gr at 2725 fps out to about 800y before they slow down enough to loose stability.  They do seem to completely loose it somewhere after 750y, 'cause they drop like a rock from 750 to 1k (more than they should otherwise)

Some of us are trying a 1-6 twist AR barrel and it shoots 55gr fmj just fine.  No dissentigrating bullets yet, but we haven't done a ton of testing with thin jacket varmint bullets sub 50gr.

Every barrel will have its own preferences for bullets, and they often don't correlate to twist or weight.  Some fast twist guns like some 55gr loads, and some 1-9 barrels are going to like 75 or even 77gr loads.  Chamber shape, bullet shape, seating depth, and other factors can play into a barrels preferences.  There's no good way to guess ahead of time what a particular barrel will like except testing similar barrels from the same lot.

The rotational speed isn't lost with distance, the need for more rotation is a product of the lower velocity that comes with the extended flight path.  

Bench rest 100/200 yard rifles use close to the minimum twist rates required to stabilize the light weight flat base bullets. They don't need or want any more than "enough". Palma (limited to 155.5 grain or lighter .308) started out using 1/13 decades ago because they were forced to use issued ammo and slower twist rates shot lower grade ammo slightly better.

Lots of people shooting Palma nowadays are running faster than 1/13 twists. Bullet quality has improved, people are shooting heavier bullets which require a faster twist rate. Very few people are shooting 1000 yards with a .308 with anything slower than 1/11.25 nowadys. Even in strictly Palma (<155.5 grain) matches. This is in part the result of better bullets being used and the desire on the competitors part to have a rifle that will work with heavier bullets. Many are running 1/9 twist with 200+ grain bullets in .308 F-TR. .

Inside 600 yards none of this matters. It's the longer distance that needs a little more twist. It's always better to err on the side of more twist when choosing a barrel.  

So, your cheap, government-profile barrel has magical properties that miraculously transform bargain-bin ammunition into ammunition that delivers consistent 1.2 MOA accuracy?

Your cherry-picked, 5-shot groups don’t mean squat for determining the accuracy level of ammunition.  Lake City’s own testing, using machine-rested, bolt-actioned, heavy test barrels shows that genuine M855 can barely hold 3 MOA accuracy on a good day.  









My own testing for the thread reposted below, using bench-rested semi-automatic AR-15s with expensive Colt heavy barrels, showed that even the best M855 (from IMI) was only capable of averaging 2.4 MOA, while Federal/Lake City XM855 averaged closer to 3.7 MOA.  I’ve tested different lots of Federal/Lake City XM855 from the year 2014 and every year previous to that for several years and none of them have come even close to averaging 1.2 MOA accuracy.




Attack of the (M193) Clones


clone:  one that appears to be a copy of an original form.

Genuine M193 must be tested for and pass all of the specifications laid out in the mil-spec, MIL-C-9963.  The required areas of testing included in MIL-C-9963 range from velocity, accuracy, chamber pressure and port pressure to waterproofing, temperature stability, bullet extraction, case hardness, fouling and more.  

Genuine US Military M193 can no longer be sold to civilians, thanks to the Clinton Administration.  The ammunition that is sold on the commercial market with some form of “M193” in its nomenclature is often referred to as an “M193 clone” because it “appears to be a copy” of genuine M193, but we generally have no idea what specifications of MIL-C-9963 that this ammunition has passed, or has even been tested for.

M193 is loaded with a 55 grain FMJ bullet with a cannelure.  The bullet itself, must meet required specifications to be used in genuine M193.  For example, the specification for the thickness of the gilding metal jacket of the bullet is 0.021" with a tolerance of - 0.002".    For comparison, the jacket of Hornady’s 55 grain FMJ bullet has a thickness of approximately 0.028”.  Jacket thickness can have a significant effect on terminal ballistic properties, particularly that of fragmentation.  Even the composition of the copper alloy used for the jacket and the lead used for the slug must meet mil-spec requirements for genuine M193.

Genuine US Military M193 can only be charged with powder that has been specifically approved by the US Military for use in this cartridge.  If the ammunition in question is not loaded with one of the approved powders, it is not genuine M193 and naturally we have no way of determining what powder was used in a load simply by visual inspection.

Genuine M193 will have the annealing iris visible on the shoulder and neck portion of the case.  It will also will have crimped and sealed primers. Genuine M193 has a crimped case mouth along with sealant at the case mouth.  


The velocity specification for M193 as cited in MIL-C-9963F states:

The average velocity of the sample cartridges, conditioned at 72 degrees, plus or minus 2 degrees Fahrenheit (F), shall be 3165 feet per second (ft/sec), plus or minus 40 ft/sec, at 78 feet from the muzzle of the weapon.  The standard deviation of the velocities shall not exceed 40 ft/sec.

The specification is for a 20” barrel.  Depending on multiple variables, this velocity specification equates to a muzzle velocity of approximately 3270 ft/sec, plus or minus 40 ft/sec.

I chronographed four different M193 clones back-to-back for comparison.  All four of these loads are currently available on the commercial market (at the time of this writing).  These loads were fired from a semi-automatic AR-15 with a chrome-lined, NATO chambered 20” Colt M16A2 barrel.  The four loads are listed below:

IMI M193
American Eagle Tactical M193
Privi Partizan (PPU) M193
Winchester Q3131A1.











M16A2 barrel.



Chronographing of the M193 clones was conducted using an Oehler 35-P chronograph with “proof screen” technology. All velocities listed below are muzzle velocities as calculated from the instrumental velocities using Oehler’s Ballistic Explorer software program. All strings of fire consisted of 10 rounds each.









Each round was single-loaded and cycled into the chamber from a magazine fitted with a single-load follower. The bolt locked-back after each shot allowing the chamber to cool in between each shot. This technique was used to mitigate the possible influence of “chamber-soak” on velocity data. Each new shot was fired in a consistent manner after hitting the bolt release. Atmospheric conditions were monitored and recorded using a Kestrel 4000 Pocket Weather Tracker.





Atmospheric conditions.

Temperature:  79 degrees F
Humidity:  42%
Barometric pressure:  30.19 inches of Hg
Elevation:  950 feet above sea level






The accuracy specification for M193 cited in MIL-C-9963F is as follows:

The average of the mean radii of all targets of the sample cartridges, fired at 200 yards, shall not exceed 2.0 inches.

These averages are from 10-shot groups fired from machine rested, bolt-actioned test barrels, such as the ones pictured below.  All things being equal (which of course they seldom are) this specification equates to a mean radius of 1 inch at 100 yards.

(For those of you not familiar with the mean radius, see post #3 in this thread.)











I conducted an accuracy (technically, precision) evaluation of the same four M193 clones that were chronographed above, following my usual protocols.  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. 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 barrel used in the evaluation was free-floated. The free-float handguards of the rifle rested in a Sinclair Windage Benchrest, 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. The set-up was very similar to that pictured below.






The Wind Probe.





The test vehicle for this evaluation was a 16” Colt HBAR with chrome lining, a NATO chamber and a 1:9” twist. This is the barrel found on the Colt 6721 carbine. This barrel was free-floated with a 10” LaRue free-float handguard.  I specifically choose to evaluate the accuracy this ammunition using an AR-15 with a chrome-lined, NATO chambered barrel, as this is the type of barrel that is most commonly used to fire this type of ammunition.  It is sometimes possible to obtain  slightly better accuracy from mil-spec/NATO pressure loads by firing them  from an AR-15 that has a stainless steel match-grade barrel with a hybrid chamber such as the Noveske NMmod0 chamber or the Wylde chamber for examples;   but you're not going to make a silk purse out of a sow’s ear.










The 16” Colt HBAR is one of the most accurate “off the shelf” chrome-lined, NATO chambered AR-15 barrels that I’ve evaluated.  Three 10-shot groups fired from this barrel from a distance of 100 yards using match-grade hand-loads topped with Sierra 52 grain MatchKings had extreme spreads of:


0.85”
1.14”
0.88”

for a 10-shot group average extreme spread of 0.96”.  Over-laying the three 10-shot groups on each other using RSI Shooting Lab software produced a 30-shot composite target with a mean radius of 0.32”.




IMI M193





Three 10-shot groups of the IMI M193 were fired in a row from a distance of 100 yards from the Colt 16” HBAR.  Those three groups had extreme spreads of:

2.83”
2.77”
2.80”

for a 10-shot group average extreme spread of 2.80”.  The three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group.  The composite group had a mean radius of 0.97”.


The smallest 10-shot group.






The 30-shot composite group.







American Eagle Tactical XM193




Three 10-shot groups of the American Eagle Tactical XM193 fired in a row had extreme spreads of:

3.01”
3.25”
3.57”

For a 10-shot group average of 3.27”.  The 30-shot composite group had a mean radius of 0.98”.


The smallest 10-shot group.





The 30-shot composite group.







Prvi Partizan M193



Three 10-shot groups of the Priv Partizan M193 were fired in a row from a distance of 100 yards.  The groups had extreme spreads of:

2.72”
3.89”
3.74”

for a 10-shot group average extreme spread of 3.45”.  All three of these 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 1.01”.


The smallest 10-shot group of PPU M193





The 30-shot composite group.






Winchester Q3131A1





Three 10-shot groups of the Winchester Q3131A1 load were fired in a row. The extreme spreads of those groups measured:

2.95”
3.73”
3.35”

for a 10-shot average extreme spread of 3.34”. 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 1.05”.


The smallest 10-shot group.





The 30-shot composite group.







Here is a summary of the results of the accuracy evaluation of the four M193 clones.










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










To be continued . . .




…….


Attack of the (M193) Clones, Part Two:  M855 Comparison.






In Attack of the (M193) Clones, part one, we examined the velocity and accuracy (technically precision) of four different M193 clones currently available on the commercial market.  In part two, we’ll be examining the velocity and accuracy of four different M855 clones currently available on the commercial market.  The M855 clones examined for this article are shown below.

IMI M855





Winchester Ranger M855






Prvi Partizan M855





American Eagle XM855






The obvious difference between the projectiles used in M855 and M193 clones is the weight difference of 7 grains, 62 grains versus 55 grains.  The M855 projectile has a FMJ construction but also has a steel “penetrator” in the ogive section of the bullet.  This makes the projectile unusually long for its weight, as well as giving it a lower specific gravity.  Most, but not all, of the M855 clones have the tip of the bullet painted green.

M855 versus M193 projectiles.





The 62 grain M855 projectile is actually longer than the heavier 69 grain Sierra MatchKing.





Note the steel penetrator in the sectioned projectile below.





Chronographing of the M855 clones was conducted as described in  part one, using the same 20” M16A2 barrel.  The results are shown in the table below.





Accuracy testing of the M855 clones was also conducted exactly as described in part one, using the same free-floated 16” Colt HBAR fired from my bench-rest set-up at 100 yards.  As previously stated, I chose a chrome lined, NATO chambered barrel as the accuracy test vehicle as this is the type of barrel that these clone loads are most likely to be fired from.  It is sometimes possible to obtain  slightly better accuracy from mil-spec/NATO pressure loads by firing them  from an AR-15 that has a stainless steel match-grade barrel with a hybrid chamber such as the Noveske NMmod0 chamber or the Wylde chamber for examples;  but you're not going to make a silk purse out of a sow’s ear.




Three 10-shot groups of each load were fired in a row with the results shown in the table below.  






Here are the previous accuracy results of the M193 clones along with the M855 clone results for comparison.





MK262 and MK318 Mod 0 thrown into the mix for additional comparison.





....

 

...

True that. My 20" 1/9 will hold 4.2 inches @ 300 yards with 77 Sierra's (although they need to be driven with a fair degree of enthusiasm to stabilize). However, at 500 yards the impact points cover approximately one county.

I offer no explanation for my results beyond stating that I only shoot 5 shot groups, the 1:9 gov't profile NATO chambered 16" barrel is nitride, not chrome lined, and that no 5 shot groups from that lot have exceeded 2" at 100 yards when shot from bench.  Maybe my sample of one barrel and one lot of true LC is not representative.  I actually thought a better bench scope (I believe those early groups were with a Leupold LPS with 10x top end and relatively fine duplex reticle), but later groups are with a Leupold Patrol 3-9x40, and are still 1.2" at a 100 yard bench.  I do not have a fixed power bench scope, but might play with mounting a Burris Black Diamond 6-24x50 with sunshade and fine target reticle to reduce aiming error and see what happens. Its the best "target" type scope I have available.  Other lots of LC pre '13 and from other "bulk" manufacturers are nowhere close to this result and are like your data, so I really don't think it is the barrel.

I fully understand the difficulties presented in making the M855 projectile accurate, given the bi-metal, steel penetrator insert construction.  I only report my limited, yet consistent, experience with this lot.

Perhaps mt cheap barrel (CMMG nitride) was just a pretty good one