The idea is as follows: put little engravings of serial numbers on the tip of the firing pin, so that every time the gun fires a round, the casing has a little serial number stamped on it for the police to trace to the source. Sounds great, but only proves Gun Controllers aren't very bright.
The problems are: (1) the technology doesn't exist (2) if it did exist, it's easy to defeat (3) most guns aren't used by their owner, don't eject casings, and were pre-law -- so this can't do anything for crime, but it's a heavy cost/annoyance to gun owners for something that can't get positive results. It's laws like this that prove to the clueful that gun legislators can't be trusted.
A little more details:
(1) the technology doesn't exist
California passed this law, because they saw it on a science fiction movie and demanded the gun manufacturers all modify their guns to be in compliance in order to come on-roster (available for sale). Only the technology to do this at scale, doesn't exist. You have to one-off manufacture these things to get it to work, in theory. This of course has resulted in a constitutional challenge for requiring something that doesn't exist -- and is costing the state millions in legal defense, for their annoying harassment of legal gun owners.
Even if you could make it exist, an exorbitant cost, on a one-off basis -- it can't stand up to regular use. This printing is very small/fine, that means it wears really quickly (even if it is a hard material). You can't engrave something micro-small on a little hammer or pin, and then smack it against metal casing, a few thousand times (once for each round) and have that molding/printing not disappear. So it's technically not possible: any material that's hard enough to not flatten, is too brittle and would shatter (like diamond). Anything else, is too soft to take more than a couple boxes of ammo before it's gone.
(2) if it did exist, it's easy to defeat
Imagine this, you dry fire (fire without a round in the chamber), against a center punch or diamond scribe. The imprinting would be destroyed with a single click. That's nothing, people would make special dummy bullets you could fire against to wipe out the engraving. If that doesn't work, guns are meant to be taken apart to clean -- so you just rub the end of the firing pin with sandpaper or a nail file 3 times, and the micro-stamping is gone. Or you just order a replacement pin/hammer from out of state and drop it in. Guns are tools, and meant to be easy to take apart and tweak.
About 30% of pistols manufactured since 1980's have been revolvers (more historically), but they're especially popular in crimes, and there were a lot higher percentage. (This isn't counting single shot, break, bolt action, and so on). These kind of guns don't eject rounds (casings) after they've been fired, so there's absolutely no benefit to having them micro-stamped. Of course California didn't differentiate between where it made sense or not -- proving the intent isn't about catching criminals, but burdening gun makers.
Oh, and even in it's primary purpose, catching rounds from semi-automatics, they make little bags as casing catchers and cheap attachments (or you could make one with some duct tape and a garbage bag or sock). You have to be a special kind of stupid to think this is going to stop or slow anyone.
(4) Criminals don't use their own guns
97% of guns used in crimes, are used by their owner thus tracebacks would matter, because it would never get to the actual owner. Even when it does, they just claim they lost the gun, and you have to prove they didn't... especially if they just threw the gun away after the crime.
(5) There are 500M+ guns without microstamping
Even if you pretend this is a brilliant idea, and you thought it should be a law -- California is ignoring that all of the guns out there, do not have this. It would be 50-100 years before there would be enough of these would proliferate (assuming all the criminals are too stupid to dry-fire a gun), with the requirement, to catch anyone.
In the end, it's ideas like this, turned into laws, that results in the informed not liking gun controllers. It's not just that gun controllers are ignorant, it's that they're evil liars. No one who is informed could support this law as being a viable way to help with crime or gun crime. But virtually everyone with a clue can see how this law could be invented to harass legal gun owners.