What stops the firing pin??

[G.T.]

Hello to all, I have been pondering a question about Luger firing pins all day and thought I might ask other forum members ... What stops the firing pins forward movement?? Is it the outside dia.?? Or the gradual taper?? Can't be the primer, although there seems to be relief for punctures?? Why are they so vunerable?? Seems to me, the fit between firing pin and breechblock would have as much to do with trigger tuning as the fit between firing pin and sear bar?? Just a thought... Best to all, til...lat'r...GT