In a 16" barrel, .45 ACP had virtually no gain over a 5" barrel. The point of diminishing returns is somewhere around 9" if I remember correctly (maybe it's 12"), the velocity actually starts to diminish with any barrel longer than that. I'm getting about 940 fps from Federal American Eagle and about 980 fps from my handloads. You also have to consider the massive losses from a gas system or blowback action.
My DI AR is an upper and magazine well adapter made by Macon Armory. The magazines are HK USC, single-stack, and they insert at the same angle as the do in the intended host, angling forward at the bottom. Note the thumb tab in the mag well adapter in the photo, exactly like the USC but not like the AR. Every time I let someone else shoot it they eject the whole adapter via the standard AR mag release and hand it to me with a puzzled look. Other than the RMW Extreme uppers, this is the only production direct-gas impingement .45 ACP I know of, and it is far, far better to shoot than any of the blowbacks. My rifle weighs 5.5 lbs loaded and has quite a bit less recoil and bolt slam than the straight blowback guns. I couldn't warm up to the RMW modified Uzi magazines, so I opted for the (then brand new) Macon system with the USC magazines. Last round hold open was a priority for me, and the USC magazines, together with the Macon adapter, work flawlessly in this regard. These two companies were making this stuff before most of the pistol magazine lowers were available, and utilize standard AR lowers. Macon makes a Glock-lower compatible AR upper, and some others. My hesitation with the Glock magazine lowers is the bolt hold-open. It looks like CMMG has it worked out according to Sooch's video, but the pistol magazine follower is weak and when an AR bolt catch starts to get dirty, things can bind up.
