Actually, all of the A2's were/are cut for 5/8-24, however the 3rd model and 12th model cans required a different barrel contour that required the barrel to be turned down in the front and the forming of a 20 degree shoulder for the can to center on. That said, both of these still had 5/8-24 threads on the barrel, the 12th model had a muzzle brake installed over the threads. When we go AAC, we got rid of the barrel contour and just threaded the end of the barrel 5/8-24 to take the AAC flash hider. Some may wonder why I used the flash hider over the muzzle brake from AAC when I used the brake from OPS. Well, this is because OPS did not have an actual flash hider although Phil (Seaburger owner of OPS) swore his brake acted as a decent flash hider but I would disagree. His brake did/does work ok, nothing too great. Also, the AAC flash hider was/is pretty effective and from my view point the A2 did not require braking at the muzzle as 7.62 doesnt really recoil all that much, however the users really needed a good flash hider when the can was not on. On top of this, it was much easier to manufacture the gun/barrel by installing the hider bc it does not have to be clocked. Lastly, we originally used the AAC 51 tooth FH but discovered that the cans would "back off" during use 1 tooth. We then went to a 72 tooth (I believe it was 72, maybe 71 or something) which was supposed to help but didn't, then AAC came out with a 90T (and did away with the 72T), however it still backs off 1 tooth which is a whole other discussion.