The reality is most gun owners aren't doing anything of the sort. However, if this stuff is implemented the purchase of the barrel and firing pin would be noted. They could track it back to the place where it was purchased and track the receipt as before.
Everybody is all about how it can be circumvented by the bad guys, and I noted that. Bad guys will change the gun so that this won't be useful. Most murders, however, aren't done by those people however much people think they are. It will be useful to catch the regular schmo more quickly.
It won't be useful for gang killings, etc. or long-term planned killings, but neither of those are the bulk of murders in the US.