Problem with S&W MP 40cal

[2ndAmendmentNut]

A friend of mine brought a S&W MP in 40S&W to me to have a look at. I am not a gunsmith but most people know me as the local gun nut. Anyways the gun has an intermittent problem with simply not firing. He told me it would feed and you could manually cycle ammo through it fine, however sometimes you pull the trigger and simply nothing happens. I am not real familiar with these guns (being personally an XD fan) but I field stripped it and had a look. The gun was pretty clean and not overly oiled and appeared to be in fine shape at least as far as I could tell, also I could not get the gun to duplicate the problem. I suggested that perhaps the magazine safety was not functioning as it should and referred him to youtube where you can find all sorts of great videos on how to remove the stupid safety. You all have any other suggestions or perhaps some things I could look for?

[rintinglen]

You might want to take a trip with him to the range and watch him shoot to see if you can spot a flaw in his technique. My SIL was having problems with his PARA ORDNANCE some times being real hard to shoot. What was happening was that under recoil his thumb was bumping the safety "almost" on, which was enough to interfere with the trigger pull. Changing his grip cured the problem. While I do not suspect that this is exactly the problem, you may see something unusual. When a gun works for one person and not another, I usually suspect operator error.

[thegreatdane]

check the striker and firing pin channel for gunk.

[Frosty Boolit]

I had one of these and it had the mag disconnect and sometimes my thumb would release the mag while my grip would keep it from falling out. Reseat mag and then it's fine. I sold that one. mag safety, who needs 'em.

[Ewald]

First, there is a known issue with the mag catch on early model M&Ps. Basically they can drop mags when you don't want them to. If that is happening call S&W for an RMA. I did that with mine and had it back a week later. No mag problems since.

Secondly, have you tried different ammunition? A friend of mine recently cooked up a batch of cast 40 S&W for a few of us with 40s. They ran fine in another friend's Glock but not so much in a third friend's M&P. The problem the M&P friend described was exactly the same as what you are describing.

I lent the bullet maker friend my M&P to experiment with. What he found was the AOL needed to be slightly longer than what he was loading for. I had him make me a batch of the new load, and they run fine in my gun.