Colt carbine 1/7 twist vs 1/9 - practical significance?

[IMtheNRA]

I was looking at Colt's web site and they list two similar products, 6920, which is an M-4 look-alike with a twist of 1 in 7, and the 6921 without the grenade launcher barrel profile, with a 1 in 9 twist. Both barrels appear to be the same length, 16.1 inches.

While I understand that twist rate has an impact on optimal stabilization for different weights of projectiles, I was wondering when one rifling rate is better than the other. Specifically, in the case of these carbines which would usually be used with the common military types of ammo.

 

[Bartholomew Roberts]

The 1/7 twist can sometimes have issues with really thin jackets diseintegrating. Usually this is only applicable to lightweight varmint bullets of 45gr or less and not even all of those; but just those with thin jackets.

The 1/9 twist can handle the lightweight varmint bullets well; but may not handle the heavier 70gr+ bullets as well, particularly in shorter length barrels. To use one example, my 16" 1/9 HBAR from Bushmaster would do about 4-5" groups at 100yds with Black Hills 75gr match ammo. It would still hit a silhouette target OK; but I wasn't getting anywhere near the potential out of the 1/9 that I would get with a 1/7 and that load.

 

[Gary G23]

http://www.ak47.net/forums/topic.html?b=3&f=118&t=242568

 

[Lone_Gunman]

What ammo weight is the 1/9 twist optimized for (in terms of accuracy)?

 

[oneshooter]

62gr/SS109 is what shoots best in my 1-9 Bushie. Preferably IMI, but will shoot well with any other but Olympus.

Oneshooter
livin in Texas