I bought one of these uppers to run on an Armalite lower (10.5's aren't exactly falling off the trees these days) - somewhat hesitatingly I took it out last week with 100 rounds of Fiocchi for a function test. It worked very well, but when I took it home and took it apart I realized I was running a standard carbine buffer - I'd probably have put in an H if I'd been thinking about it (and I'd probably have had issues). Ejection was clean and smooth, cylce was as smooth as any AR I've shot, brass 4 feet at 4 O'clock, and the little bugger was dead on at 200m on a steel plate with irons. I'd certainly recomend a carbine buffer in these uppers, and leave the M@^*&% F^&*%$# S*&% ammo on the shelf where it belongs.