Stern ar9 upper receiver - Stern Defense SD Mod4 M-Lok 9mm Complete Upper Receiver

Was pleasantly surprised to find the quality of the upper to be better than expected. Fit/finish was excellent and dropped perfectly on to both my Daniel Defense and Anderson carbine length pistol lowers without issue. Paired up with their Glock magazine adapter in the lowers I have run over 300 rounds through reliably with last round bolt lock for every mag. Ran 115, 124 and 147 grain rounds and all have cycled perfectly with or without a can. Have not tried with a super safety but 100% works with cassette style FRT’s. Paired with red springco spring and stiffer hydraulic 9mm buffer it’s a blast and still very controllable. Just don’t expect recoil to be like an MP5…