I got a used Police Issue M&P .40, 4.25" from AIM Surplus.

Looks great. It doesn't seem like anything is broken, however, when I took it to the range twice the slide would lock back about 1 out of 3 shots.

I have looked on the internet, but have not found the right answer to my specific problem. So hopefully someone here can help.

Are you accidentally placing your thumb on the slide lock while you are firing? Where are your thumbs placed on your grip? Pics are always helpful.

I don't think that is the cause. It has happened with my brother and my father shooting it.

I forgot to mention, there is an unusual carbon line and bulging in the middle of some of my spent brass casings. However, I was not able to determine if those casings were the ones extracted when the slide locked back.

I love the pistol, and when it doesn't lock it shoots beautifully. It is my first more modern pistol. My only other experience is with my CZ-82.

Can you post pics of the casing? There are a lot of informative members on this site. Pics always help. I doubt that is causing you your issue. Also show a pic of your grip on the pistol. Although you did have someone else shoot the pistol, it is possible that they placed their thumb on the slide lock. Being a used police M&P, the slide lock could be very easily manipulated,

Pull your slide off and just see if there is any spring tension on the slide stop lever holding it down. If the gun has been apart they may have not got the lever spring back in place correctly .

These guns may not see a lot of shooting, but they do see a lot of handling. So, the suggestion to check the slide stop spring tension is a good one.

Here's a little help with parts identification if you're new to the M&P:
M&P Parts Identification

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Here is a video of my brass and my pistol w/o slide.

M&P 40 Malfunction - YouTube

I will double check, but the slide stop should not move when the trigger is pulled.

It looks like you don't have a slide stop spring installed at all.

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This might help:



The red arrow is pointing to the slide stop spring. It's yellow in my gun and you can only see the one arm of it. The rest is under the trigger block. Look to see if this spring is in the little hook on the slide stop. It could be in the gun and just not on the slide stop hook designed for it.

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I watched your video. To answer your question "is it supposed to do that"? It is not supposed to do that.
What is the status of the spring mentioned in earlier post? It maybe missing or misaligned.

I have checked and the spring is where it is supposed to be according to the image shown above.

I know that mechanical moving parts can and do wearing out. How prone to this are these springs?

Where can I get a replacement if needed?

Also, what brand of ammo are you shooting? I'll get constant FTE's when shooting Fiocchi ammo. Don't know why that is.

Yours is the first gun I've seen that does that. Of course that doesn't mean it's the only one.

Here is where you can get a replacement slide stop: M&P Slide Stop Assembly, Ambi [1390] - $12.90 : Welcome to Speed Shooter Specialties

Even without the spring, the trigger should not move the slide stop like that. Something is rubbing on something it shouldn't.

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UPDATE:

Took my M & P .40 to Anthony Arms in Pittsburgh, they have a gunsmith there on Wednesday thru Sunday. It was the Slide Stop Spring. It was worn out.

It appears to me as though yo may have a missing or broken pin somewhere.