How does ammunition deliberately leave an identifying trace?
Micro stamping is currently not enforced strictly but if it would it would be no more legal to swap the firing pin as it would be to put in a full auto fire control mechanism on a federa level, or on a state level flash hider or a normal capacity magazine in states that forbid them.
The stamps are required to be on the firearm, not the ammunition. They're supposed to leave an identifiable mark on the ammunition when fired.