I have a new full-size M&P .40 S&W. I like the idea of having the option of two calibers in one pistol, so I bought a 40-9 conversion barrel from KKM Precision.

The conversion barrel did not work reliably with the .40 S&W magazines. The magazine wouldn't hold the 9mm rounds securely. Inserting the magazine was more difficult, cases ejected erratically and the slide wouldn't lock back after the last round.

I miraculously bought 2 new 9mm magazines at a reasonable price from a local individual. The conversion barrel works perfectly with 9mm magazines. Ejection of cases was consistent and out of the way.

It seems the M&P series is particularly well suited for a 40-9 conversion barrel since the extractor and ejector are the same for both calibers. The fit and finish of the KKM barrel is excellent. Easy drop in fit. Well worth the money. I'm pleased.