On a whim I bought 2 of these and my son and a friend bought 1 ea. All four rifles have the same brand upper, but each has a diff brand lower. 2 worked just fine,2 needed a little rework of the m4 feed cuts. On the 2 that needed work, the bullet nose was hitting slightly low going into battery, not quite 1/16". With fmj, no problem. With soft point it would feed fine, but would shear off part of the lead nose. After smoothing the transition point from the m4 feed cuts to the bbl extension feed ramps all work great. Just shows to go ya that there are very small differences between lower manufacturers. Not a perfect mag, but not too bad.