wacki
Member
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