Twist rate on 5.56/.223 caliber rifles has been one of those issues that gets debated in a lot of discussions I have with civilian and law enforcement shooters. Without getting deep into the history on twist rates, what you need to know is that those old M16A1 rifles that some agencies have been getting from the U.S. Government are going to have 1/12 inch twist barrels — this works well with 55 grain bullets but not much else. When the U.S. Military adopted the M16A2 in 1980’s it, along with most of NATO, switched to 1/7 twist to stabilize the heavier rounds they were using. Some countries and commercial manufacturers went to 1/9 as an alternative twist rate that would work equally well with 55 grain and 62 grain bullets.
What we see today in the LE patrol carbine market is mostly 1/9 or 1/7 twist barrels. So which one is better? For me and my agency it was clear: 1/7. The reason is that the largest consumer of small arms ammo is the military. As such it exerts a great deal of influence over military caliber small arms ammunition development. Two of the best rounds developed in the last few years in 5.56mm have been the 75 grain TAP T-2 and the 77 grain Black Hills Mk. 262. Both of these rounds were developed to work well in 1/7 twist barrels specifically. Accuracy is either inconsistent or nonexistent in 1/9 twist barrels.
If you want to take advantage of the effective heavier rounds currently being offered and developed then you need to run 1/7. I have seen 1/9 barrels that function very well with the 70+ grain bullets. I discussed this issue with Giles Stock at a Hornady ballistics class. He explained that you can take a sampling of rifles with 1/9 twist barrels from the same lot and some will work well with the heavier bullets and others will not. The reason is that during the manufacturing process some are cut closer to 1/8 and some are closer to 1/10. The ones closer to 1/8 will stabilize the bullet better and you will get better accuracy.
My agency used Hornady 55 grain TAP as our duty round until 2010 when we began switching to the 75grain TAP 5.56mm T-2. The T-2 gives us better penetration after impacting an intermediate barrier and auto glass yet still fragments reducing potential over penetration. Our training ammo has always been 55 grain FMJ. I have not observed any loss of accuracy or any other issues when training and qualifying with 55 grain rounds in our 1/7 twist barrels and we have not seen any change in POA/POI between the 55 grain and 75 grain rounds. At range sessions when we shoot at longer distances we shoot previously issued duty ammo and replace it with fresh ammo. By using duty ammo we eliminate the possibility of any measurable POA/POI deviation to interfere with training. At ranges of less than 50 yards, any difference is unperceivable. Therefore, we have been able to take advantage of less expensive and more abundant 55 grain rounds for training and field arguably the best duty round for our agency.
When you realize that any twist rate other than 1/7 for 5.56/.223 caliber rifles is only going to serve to handicap your department’s ammunition choices now and in the future, it becomes difficult to argue for something different. These rifles will be in service for decades after you have retired so make sure they will be able to take advantage of future ammunition developments.