My son (25) bought 10 of these mags, thinking he could tweek them if they needed it. Well they fit and function perefctly in his Bushmaster M4 clone. I tried a couple in my Colt H-Bar and they fit and functioned perfectly for me. My son then ordered 2 dozen more of these and they have all been very good mags. All seem to fit well, and the ones just randomly picked function well. Haven't found a bad one yet. Haven't shot from all of them yet either!Seem to be a real bargain in Mags.