I have a FS9 I bought back in October, I've put 1800rds through it and had the same problem I think 3 times. My kit came with 3 mags, I cleaned both the gun and mags before firing. I also marked each mag so I can tell the difference between them if there is a problem. Sorry, I guess I forgot to say, the slide wasn't staying back after an empty mag was the issue. But I did notice it was with the same mag. I've fired 400rnds since the last problem. Everything seems to be fine now.

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S&W M&P 9
US Veteran