Just got mine back from S&W for the second time for a similar problem- mine would lock back on a partially full magazine on a regular basis. Only difference was my slide actually locked back, not stick back. First time they replaced the slide lock which didn't help at all and the second time they replaced the locking block which appears to have cured the problem. Just put around 150 rounds through it at the range with only one early on lock back and no further issues. Will need to put another 100-200 rounds through it before I consider it reliable for carry.

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Sid