AR rifle twist 1-in-7, 1-in-9, overspin,... etc

[wacki]

I saw the below text on a another forum. Why is overspinning bullets bad? How much extra accuracy will you get with a 1-in-7 over a 1-in-9 on the milspec bullets?

Hi Rich,

The M16 family of weapons were fitted with 1:7 twist barrels in the early 1980s in order to stabilize the new composite core M855 5.56x45mm NATO ammunition, which was being adopted, along with the M249 SAW, at that time for US forces. Existing M16A1 rifles used a 1:12 twist rate for the rifling in their barrels, which was acceptable for stabilizing the then current M193 55-gr lead cored FMJBT round. M855 bullets have a center of mass further back on the round and this makes them more susceptible to tumbling in flight without a fast spin rate provided by barrels with tighter twist rates. As such the current M16A1 could not fire an M855 round for more than 100 yards or so without the onset of tumbling. I’m not sure the reasons why a 1:7 twist rate in particular was selected; there has been some speculation that this twist rate is a bit excessive for the M855 round. It is also possible that it provides extra gyroscopic stability for the use of other types of composite cored ammunition, existing or planned, such as the military M995 armor piercing round, etc.

So why do commercial AR15 clone manufacturers use a 1:9 twist barrel in their products? With the incredible rise in popularity of the AR-15 among civilian gun owners in the 1990s and 2000s, particularly post 9/13/2004, there existed a need for their products to fire a wide variety of ammunition with good accuracy. Commercial .223 Remington ammunition runs a wide gamut of bullet types, lengths, masses and configurations, so AR gunmakers building complete general purpose AR clones needed a rifling twist rate that would fire a jacketed, lead cored .224 bullet between 45 grains and 75 grains well. For this reason, a 1:9 twist rate works well because it will not overspin lighter bullets but can provide enough gyroscopic stability for the heavier 65-75 grain commercial ammo and thus it is used in virtually all civilian ARs. I personally prefer the 1:8 twist ratio myself, as it produces a bit better accuracy with the heavier and longer bullets.

Some manufacturers (Rock River, etc.) do offer the option of different twist rates for their rifles and these can also be sometimes custom ordered form the manufacturer. Colt used to sell all of their ARs with 1:7 twist and still do offer that rifling for their LE6920 rifles, which are the closest a civilian can get to owning an actual M4 carbine. The bottom line is that 1:9 twist is perfectly acceptable for regular target shooting and self defense with an AR, I prefer the 1:8 and the 1:7 is the current Mil-Spec

 

[briansmithwins]

My 1/7 AR has both 50gr and 55gr bullets delivering 1.5" groups at 100 yards.

It may be possible to 'overspin' bullets but I've yet to see it happen in real life. BSW

 

[cracked butt]

Its possible to cause bullets to 'blow up' if they are driven too fast with too tight of a twist rate- but this usually occurs near the end of the barrels' life span when the throat gets rough.

FWIW, my AR currently wears a barrel with a 6.5:1 twist rate which was meant for 80-90gr bullets, but shoots 52 gr bullets very well.

 

[chrissmallwood]

Im no expert so take this with a grain of salt but I was under the impression that the issue of bullets disintegrating due to too tight a twist only arose when using the thin jacketed 40-45grn varmint bullets.

 

[Jim Watson]

MY 6.5 twist .223 meant for 90 grain bullets, will blow up 75 grain A-Max in midair, if pushed fast enough to make it to 1000 yards supersonic. Fine 600 yard load, though.

Military FMJs are tougher than varmint and target bullets, and stand the extra centrifugal force better.

The quote in the OP "I’m not sure the reasons why a 1:7 twist rate in particular was selected;" is readily cleared up. The 7 twist is for the SS 110/M856 tracer bullet. A 9 twist is adequate for the ball ammo. It is marginal for 75-77 grain bullets, though, so if you want the heavies, 75-82 grains, get an 8 twist, or a 7 for the extra heavies of 90 grains from Sierra, Berger, and JLK.

The first recorded use of fast twist .223 barrels was the early effort by the USMC to make the M16 into a target rifle, with a 10 twist to add a little stability and accuracy to cheap 55 gr M193s.

Everybody is twist concious these days, and the matter is considerably overanalyzed for most purposes.

 

[RockyMtnTactical]

I think 1/8 is the ideal twist for anyone not shooting tracers (M856). I wish that were the standard.

1/7 was a compromise between the M855 round that needed a 1/8 or 1/9 twist and the M856 that needed a 1/6 twist.

 

[Larry E]

Aside from causing some bullets to come undone, a too fast twist isn't going to degrade practical accuracy for the average rifle, cartridge, or shooter. The only place it's really important is in competitive benchrest - maybe. For field use, a twist that's 'too fast' is better than one that's 'too slow' IMHO.