And herein lies a clear demonstration of the problem.
A firing pin is designed to slam metal into metal. Do you really think a detailed pattern on the end of the pin will not soon become too blurred to be useful? Even if somebody doesn't take an abrasive to it.
And it's not like a firing pin is somehow unique to the gun. They are wear items, in time they get replaced. And you have to be able to take the moving parts of a gun apart for cleaning, there's no way to ensure a given pin doesn't get swapped to another gun.
And this once again comes back to at best only being useful in the cases where the gun was legally purchased, which is a very small part of gun crime.
Testing has demonstrated that microstamping remains readable on casings after thousands of rounds with a SEM.
Moreover, even if it wasn't, who cares? It would still be an improvement over what currently exists (unintentional microstamping) given a fair number of guns that are used in crimes have barely been fired.
Nor does it matter if someone switches the firing pin since criminals aren't likely to do that. They don't even file down serial numbers most of the time.
The fact that it isn't absolutely 100% perfect doesn't mean it isn't better.
This still comes down to the basic problem that the vast majority of crime guns were not obtained by legal means. Yes, criminals don't file down serial numbers because there's no reason to as the serial number isn't connected to them.
While the vast majority aren't obtained through legal means, a surprisingly large percentage can be connected to the individual who committed a crime with one.
According to the BJS, of those in prison in 2016 who had a firearm during the offense they were in for, 7.5% purchased it legally under their own name and 25.3% obtained it from family or friends. An older 2004 Survey of Inmates of State Correctional Facilities showed 12% obtaining them from retail and 41% from family/friends.
Criminals don't file down serial numbers not because there's no reason, but criminals don't think they'll be caught. Most simply don't think very far ahead.
Micro-stamping isn't perfect, but again, it is better. If it can lead to arrests in even a small percentage of the thousands of unsolved murders ever year, it would be a huge win with little downside.
I don't believe the study asked how many family/friends obtained the firearm legally. It seems unlikely that it would be 0%.
Overall, 6.7% of prisoners reported gone through a background check when obtaining a firearm at a retail source (that's not 6.7% of the 7.5%, but rather 6.7% of all prisoners).
A "retail source" includes the 7.5% number above which is just licensed firearm dealers, but there were other retail sources I didn't include (pawn shops: 1.6, flea markets: 0.4%, gun shows: 0.8%).
Of course, the background checks doesn't really matter given that those 7.5% reported giving their actual name to a licensed firearm dealer which is what one would need to track them down.
A firing pin is designed to slam metal into metal. Do you really think a detailed pattern on the end of the pin will not soon become too blurred to be useful? Even if somebody doesn't take an abrasive to it.
And it's not like a firing pin is somehow unique to the gun. They are wear items, in time they get replaced. And you have to be able to take the moving parts of a gun apart for cleaning, there's no way to ensure a given pin doesn't get swapped to another gun.
And this once again comes back to at best only being useful in the cases where the gun was legally purchased, which is a very small part of gun crime.