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Which Twist Rate is Best for Your AR-15
Testing done by: +Andrew Tuohy 

One of the most-discussed topics regarding the AR-15 and similar .223/5.56mm rifles is rate of twist. A faster rate of twist will spin the projectile more over a given distance than a slower rate of twist, and rate of twist is closely related to projectile length and diameter.
 
This isn't really an issue with a lot of calibers, but because of the relatively narrow diameter of the .224" projectile, minor changes in weight or material can result in significant changes in length. If one chooses to shoot only the most common ammunition on the market, 55 grain lead core FMJ, the choice of twist rate is not essential - practically any .223 or 5.56mm barrel on the market will work with that bullet.

However, there is a wide variety of .223 ammunition available, and in times of short supply, sometimes one might have to buy ammunition that has longer or shorter projectiles than "normal." This could be anything from 35 grain varmint hunting ammo to 77 grain match grade ammo.

Many claim that if the twist rate is too fast, the bullet will become "overstabilized" and either come apart in midair or tumble out of control. The twist rate many blame for this condition is 1/7. If the twist rate is too slow, the bullet will yaw in midair and be less accurate than it would have otherwise been. At the extreme end, the projectile will "keyhole," or impact the target sideways. Firing 75 to 77 grain match bullets with 1/9 or 1/12 twist barrels can cause this.

While preparing for a potential LuckyGunner Labs post on the topic, I did some preliminary testing with ammunition representing basically everything on the market - from 35 to 77 grains. I fired this ammunition through 1/7, 1/9, and 1/12 twist barrels. 

All groups were fired from the bench with a sandbag for support and various scopes, all of which were set at 9x magnification. Single 10 shot groups at 100 yards were fired on days with nearly identical atmospheric conditions (temperatures in the mid-80s with low humidity).

The 1/7 twist barrel was button rifled stainless steel, 16" in length, and chambered for .223 Wylde. The 1/9 twist barrel was hammer forged chrome moly steel, chrome lined, 16" in length, and with a 5.56mm chamber. The 1/12 twist barrel was hammer forged chrome moly steel, 20" in length, with a .223 Remington chamber. 

The 1/7 and 1/9 barrels were installed in an AR-15 rifle with a free floated handguard. The 1/12 twist barrel was used in a bolt action rifle.
Again, this was preliminary testing and not the strictly controlled testing that we would normally use for an LGL post, but the results were still interesting. Due to the differences in barrel construction and action type, comparing group sizes directly is not representative of all barrels of that twist rate, but stability was what the test was really about.

The 1/7 twist barrel did not overstabilize any of the ammunition types. It was remarkably accurate with some of the lighter bullets. Although it generally shot better groups with the heavier projectiles, it was accurate and precise enough with the light bullets for almost any purpose, including varmint hunting.

The 1/9 twist barrel properly stabilized all of the lighter and medium weight bullets, but 77 grain projectiles were yawing significantly at 100 yards. 
The 1/12 twist barrel shot very well with all of the lightweight bullets. It also properly stabilized 62 grain lead core projectiles, but with 62 grain steel penetrator M855 ammunition, it keyholed. The same went for 69gr and heavier ammunition - complete 10 shot groups were not fired for 75 and 77 as the first shots were keyholing.

It appears from this preliminary testing that the conventional wisdom regarding 1/7 twist and light .223 ammunition is at least partially incorrect. Even the 35 grain bullet traveling at nearly 3800 feet per second stayed together and proved to be remarkably accurate from the fast twist barrel. 
If you'd like to see more testing on this topic at http://www.luckygunner.com/labs - please let us know.
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16 comments
 
The peaks at 55gr and 62gr bullets are probably due to lower quality milsurp ammo being compared to higher quality retail ammo. 3 MOA is still pretty nice. Just my $.02
 
Along with the inclusion of longer ranges as other commenters have mentioned, I think a 1/8 twist barrel would be an interesting inclusion as well as they have been gaining in popularity as a compromise between 1/7 and 1/9. It might turn out that 1/7 is just as accurate as 1/8 with lighter bullets, but it would be interesting to see them compared, at the longer ranges especially.
 
It would be interesting to the the muzzle velocities too.  I'm curious how fast the 77gr bullets would be moving out of the 20" barrel.
 
+spencer dawson
 Yes, the ammunition types used are as follows:

35gr Hornady NTX .223
40gr Winchester BST .223
45gr Remington JHP .223
50gr HSM MVFB .223
55gr Federal FMJ .223
62gr Federal Fusion .223
62gr Federal M855 5.56
69gr Federal SMK .223
75gr Hornady BTHP .223
77gr Federal SMK .223
 
+Phillip Strauss
 I'll check that with a 1/8 or 1/7 20" barrel, since I'm leery of firing keyholing projectiles through my chronograph...
 
+Andrew Tuohy it would also be great to see this done with 30 caliber.  MOA / RPM would be an interesting graph too.
 
Could you do this for 7.62x51 and 7.62x39?
 
The issue of twist rates is rarely significant.  Most rifles are sold with barrels with the appropriate twist rate for the caliber.  .223 Remington/5.56mm NATO is unusual in that over its history the normal range of bullet weights has changed so much that the twist rates of earlier times is not the best for some ammunition.

Hence, Andrew's very informative chart.
 
+Brian Battles It's not really relevant for those calibers, Brian, ESPECIALLY x39.  Of course, it is a somewhat more interesting question with 5.45, and interesting to note that the AK74 usually has a ~7.5" twist rate for a 53 grain bullet . . . and with high quality ammo (or even with 7n6 if you get a consistent batch), they can be remarkably accurate.
 
If you're handloading, and you can only get projectiles too heavy for your twist rate, you might try drilling a hole in the base of the bullet.

What this does, theoretically, is create an area of low pressure at the base of the bullet, making it partially drag stabilized.

I can't at the moment, but someone should try it and see how to make it work!
 
Yes, it will do that, no it won't be less stable, it will be more stable. That's what drag stabilization is, when the CoP is behind the CoG.

That's why arrows have fins in the back and weight in the front.
 
This is an excellent topic for a LG Labs test - there is so much witchcraft regarding barrel twist! You guys cleared up the steel case issue - this one would be good also - thanks for great stuff! (and good ammo too!)
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