As was mentioned, yes, it is legal. The law states that the magazine can only hold ten rounds maximum for the calibre it is intended for. People have been getting away with this for years shooting AR's and using Pmags and Beowulf mags in 3gun. The problem is though, that the feed lips are too large in the 40 mags and they will cause issues putting 9mm in them. I have tried this in the CZ TS mags, and they did not work. I have also tried this with Glock 40 mags and they too do not work. Beretta mags do work, but NOT 100% and I would not feel comfortable with the reliability, especially in a match. I was able to stuff twelve 9mm in the beretta 40 mags and they did shoot ok, again, I would not trust them for reliability, so it was for trial ONLY. The extra few rounds is not worth reliability issues!