Shield Slide Locks Open Only After First Round

[Rumbowsky]

I bought a new M&P Shield .40 with safety. I've shot about 400 rounds through it. Initially, the Shield was flawless. Lately, the Shield's slide locks open after firing the first round. I have two 7-round, and one 6-round magazines. With all three magazines, when the Shield is first fired, the slide locks open after the first round about 75% of the time. I then release the slide and the gun shoots perfectly until the magazine is emptied and then locks open normally after the last round is fired. I make sure that my thumb isn't near the slide lock when I shoot, and I make sure I'm not limp-wristing the Shield.(I actually limp-wrist on purpose on subsequent shots to see if that's the problem, and the gun fires flawlessly).
Any thoughts? Any known problems?

 

[CB3]

With the slide off the frame, push up on the slide stop to see if the slide stop spring--that holds it down--is strong enough, or broken or missing.

If the spring has enough tension to keep the slide stop down, except when a mag follower pushes it up, there doesn't seem to be any explanation other than you unknowingly pushing it up.

.40 in a small gun like the Shield has a good bump when firing each shot, but if it doesn't occur randomly throughout the mag, it is not likely a mechanical problem.

I am not aware of this bring a known problem, other than human caused.

Have other experienced shooters shoot the gun and see if it happens with them.

 

[Brucenterry]

Does this happen with every mag? If you're not sure....next time it happens, put a mark on the mag, then change to another and see if it happens again. At least you can rule that out of the equation.