Despite CA's idiosyncratic firearms laws, microstamping effectively does not exist in the real world (as in, no firearms on the market employ it, though the technology does exist).

Firing pins are pretty easy to swap out, though. Easy enough that this could be used to tamper with evidence or even frame someone. It doesn't really seem like a great idea.

Anything in a gun is easy to swap, but also illegal if it violates the law.

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.

I'm guessing they are talking about barrel rifling and firing pin/ejector marks that are used in matching weapons to crimes. I guess the could add some serial number during firing, but I've not heard of this as something in civilian weapons.

Generally speaking, it doesn't.

I know that firing pins usually leave a fairly unique trace.

I have never heard of serial numbers on bullets, and a google search led only to proposed state-level legislation to require them, which has not passed.

As far as I'm aware, no handguns currently being sold in the US employ microstamping. Also, I've never heard of serialized ammunition being sold in the US. Do you have sources on either of these currently being on the market?

I was not aware that any ammunition in the US is microstamped.

California has a law requiring the use of microstamping on semi-automatic handguns, however, models previously certified for sale in the state are exempt. I do not believe any manufacturer is actually offering a microstamped firearm for sale.

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.