after 4000 shots , firing pin broken , What could it be?

Trick question?

A broken firing pin?

Send it back to smith, they will replace it. Or call them and see if they will just send you a new firing pin.

This isn't the first broken firing pin. It happens. Doesn't have to be anything you did... Mine broke after maybe 12k rounds...

I've had 5 out of 7 that I broke in the exact same spot. Lowest round count before break was 2600, highest was 20,200.

Unless you know a certified S&W armorer close who is willing to get the parts for you, only other option is send it in for repair.

On a side note, I see that Volquartsen is making extractors for the 15-22, maybe they can make a beefed up firing pin as well.

My MP 15-22 was problematic from the start.

A bent ejector was causing a lot of jams.

Fixed that and still had about 1 misfire per mag right from new.

Eventually got a lot worse.

Finally removed the firing pin and found it broken just like in the above pic.

Had to be that way from early on.

Being a tool and die maker, I know that if the heat treat is not perfect this can happen.

Called S&W and they sent me new one.

Got it installed and waiting to test it in a couple of weeks when I go to my cabin.

Really???

S&W sent you a new firing pin??

You didn't have to send the whole weapon in for repair??

__________________
AKA: "Strange Tamer"

From Page 13 of the owner's manual:

"Caution: Never dry-fire the rifle as damage to the firearm could result."

Not saying you did, but I've known others who did...

Or it could just fall under the category of "stuff happens".

__________________
Pisgah

That is correct.

I was quite shocked.

I called with the intention of getting the ball rolling to send it back.

I figured it couldn't hurt to ask if they would send me the part.

To my surprise I was told to give my address and they would get it on it's way.

One week later it arrived.