I took a few minutes to field strip the M&P FPC so I could get an understanding as to how its bolt was made and how the firing pin was (supposed to be) retained in it.
It is simple straightforward one piece bolt with a .080” split pin going vertically through the bolt and passing through a notch in the firing pin.
The roll pin is used to retain the firing pin in the bolt’s firing pin channel while allowing the firing pin to move forward and strike the primer when the rear of the firing pin is hit with the hammer.
I would assume that there was supposed to be a spring on the front portion of the firing pin to return it to the rear position after the action cycles and the hammer is no longer against the rear of the firing pin.
I say “assume” because there was no spring in the bolt or around the floor where the firing pin fell out.
If someone who has an FPC took a look at their bolt we’d all know for sure if it’s supposed to have a firing pin spring or not.
As to how or why this happened since I wasn’t present at assembly I can only speculate on the proper assembly procedure but I suspect it is something like:
1.) Put firing pin spring on firing pin and insert it into the firing pin channel in the bolt.
2.) Align the firing pin notch to be in line with the split roll pin.
3.) Push the firing pin forward and CAREFULLY push / drive pin through the bolt to retain the firing pin (& spring??) within the firing pin channel in the bolt.
However the procedure was supposed to be by examining my bolt and firing pin I’m confident that is not how it was assembled.
The split pin is bent so much that the firing pin (big end) can be pushed into the bolt’s firing pin channel with your fingers.
I strongly suspect that the firing pin was driven into the firing pin channel after the roll pin was inserted through the bolt and it was so tight against the roll pin that the hammer didn’t have enough energy to drive it forward to the firing pin resulting in the loud “CLICK” that occurred.
I’m actually surprised I got it to fire the two times I did.
When I had it back in the gun room with the bolt in slide lock and moving the gun around to look in it and moving the bolt via the charging handle the uncaptured firing pin just fell out.
This design is so simple that it seems a person would almost have to intentionally mess it up.
To me it seems the hammer strike area of the rear bolt face seems more buggered than I would think a dozen or so strikes would leave it.
Cheers
James