From experience in manufacturing I guarantee you that those short firing pins were operator error that he or she slipped past the inspectors.
The inspectors check one part every so often, usually your first piece of the shift, then one spot check before lunch and another after lunch. It is the operators responsibility to check parts on a set schedule, usually every 10th piece.
However, Manufacturing Managers, being penny pinching bean counters, the Operator stands just as much chance of getting fired for "to much scrap" as he does for letting it go. it is not in the workers best interest to scrap junk parts, those usually are caught in assembly because they don't fit, but with a firing pin being to short, the assembler would never know unless it happened to be one of the ones that got test fired.
I had a Dept supervisor named Kenny Lantz at T/C Arms tell me "Go ahead and send the junk, we have a lifetime replacement warranty, We have to make the ship date, we can fix the customers gun later."