Springs.
Firing pins were never really a problem as such because generally we found that the FPP would increase. Yep, I know you're not going to believe it but it's due to the constant dry firing when the striker collar repeatedly beats on the bolt head tenon. As it does, it slowly increases the FPP - not much, but it does over the years. Naturally this act DEcreases the bolt head lift but as this measurement is between .016" and .060" (feel free to correct me please anyone...) it's quite large and can easily absorb the small dimensional decrease.
Mechanically and in metallurgical terms, it is impossible to shear that lug off from the rear - as in a cocking piece safety stop lug striking it from the rear. I won't go into the mechanics of it but think load and inertia. Anyway........ What WILL break it off is when the rifle goes into half-cock and a cack-handed clown ignores the basic training lesson 2 - action of the mechanism - knocks seven bells out of the bolt lever trying to free it. Grateful to Simon for the photo.