The news today has a story about the fools in New Jersey who passed a law mandating the use of micro-printing for all guns sold in the State. Evidently the thinking, if that's what it could be called, is that the scene of any gun crime will be littered with micro-stamped bullets that will lead the police to the person who bought the gun.
I think most guns used in most crimes are stolen and very few are actually owned anymore by the person who bought them. I think that micro-stamping individual characteristics on each gun won't help much and probably not at all after shooters start scattering the scene of the crime with bullet casings they scooped up at various pistol ranges.
I used to shoot at the San Diego police range in San Diego all the time and at Duffy Town and yes we always made an effort to recover the spent brass but one could always find stray casings from previous shooters. I imagine shooters in New Jersey routinely do the same thing whenever they head out to the range.
On the gripping hand? There are something like 400,000,000 firearms in the possession of various people living in America. Micro-stamping isn't a retroactive possibility even if it ever becomes a reality. Right now the technology to do it doesn't exist.