OK, I went back and watched the last scene again. I think I may have initially misunderstood what Wahlberg's character meant when he said he swapped all the firing pins. I thought he meant he had swapped them between his rifles. I could not see how that could work; these rifles are pieces of precision machinery. Sapping over the firing pins might stop one from firing because its firing pin would be marginally short, but that means the other rifle would now have a marginally long firing pin and would therefore still fire.
It would only make sense if he swapped all the firing pins for different (shorter) ones; just short enough to not fire, but not so short that you'd notice without measuring.
