Good little rifles. Not alot that goes wrong with them.
Same lockup & slide action principle used on the centerfire Model 25 and the large frame Model14.
Butt stocks from the M12 fit the M25 IIRC.
Model 12 firing pins get work hardened and break especially if they are a bit too long and are hitting the edge of the chamber on dry fire. They should just clear that by a few .000".
Even then, dry firing these and most .22's is not a good idea.
The M12 firing pin is also it's ejector.
It gets a hit from both directions with each shot and ejection. The shape of the part is not all that great for the job it's directed to do. Fragile design ? We're looking at rifles that are close to 100 y/o. Things do break down!
Field strip as far as removing the bolt itself is about the extent you normally want to take that exercise.
If you do decide to remove the slide action bar/assembly out of the rifle,,the cartridge retainer is a separate piece that will fall free once the action bar is pulled out of the frame (to the rear).
Many of these Model12's have that little part (cartridge retainer) missing from such takedowns by prior owners.
It can be seen (or not, if it's missing) with the rifle assembled, but it's not something that jumps out at you.
If the loaded rifle wants to feed more than one cartridge out of the magazine and onto the carrier, jamming it,,first thing to look for is if the cart. retainer is in the gun.
I bought two Model 12C rifles from a shop that closed down in the late 80's. $35 a piece. My brother still has one and wouldn't part with it. I sold the other. Shoulda' Kept It!