I've had a total of 4 S&W magazines for my M&P 40. I had to replace springs on 2 after 300 rounds and the other 2 had to be replaced immediately.

The two that came with the pistol both started letting the 13Th of 15 rounds nose dive enough to catch the case mouth on the front of the magazine and jam the gun. Both had just over 300 rounds through them in about 6 months time. Replacing the springs fixed the problem.

The two mags i got from the rebate program immediately had a problem I've seen people complain about many times. Which is...After shooting a magazine to empty and the slide locking back automaticly...the slide would fall on it's own when inserting a fresh magazine. Many people say this is not a problem but this is most definitely not how the pistol is supposed to operate!

I've found the problem and solution...at least in my case...The slide stop was not being pushed upward enough to securely hold the slide...it would grab just the edge of the slide stop cutout on the slide and then the action of seating the next magazine jarred the pistol enough to allow the slide to slip off the stop and chamber a round.

The springs were about an inch shorter then both the original magazines and the replacement springs I had. Replacing the short springs allowed the slide stop to travel fully up wards securely holding the slide and completely fixed the problem.

My experience with Smith & Wesson springs have been a bad one. To be fair S&W did say they would send me a return tag for the original magazines...I replaced the springs on my own and didn't take them up on it.