From February through July (with updates into December) 2008, we ran a series of articles on "Microstamping", as exemplified by Todd Lizotte's "Pie In The Sky" scheme to offer this unproven technology to ... well, essentially, to states who wanted to so impede the ability for citizens to purchase firearms at an affordable price. This series includes a Q&A email interview with the progenitor of the 'technology'; Todd Lizotte
(You can read the entire series here; unfortunately it is in reverse order by date, so the last is first.)
Recently, California decided to "Bite the Bullet". (In this case .. to byte the casing, or the primer.) In 2007. the California Assembly passed a law which required that all new firearms purchased in California "microstamp" unique data on the primer (via the firing pin) or on the face of the cartridge case (via the breech block) to identify the gun(s) which fired the bullet.
Never mind that the firing pin may be replaced by a generic model without the 'microstamping', or the slide; or that a casing which has been fired by many similarly 'microstamped' firearms may have overlapping imprints which preclude accurate identification of firearms similarly equipped.
(Note that while the primer is unique in each firing, firing pins which are legally required to carry the unique identifiers are prone to error; the cost of "replacement firing pins" bearing the same legally required identification is absurdly expensive and is prone to being 'avoided' by simple means. Firing pins are "consumables", and cheap without microstampings; they may be readily purchased from outside the restrictive state without microstampings.)
Also, the casing may have been 'picked up' on the range and may thus carry multiple microstampings .. each of which might overstamp each other, rendering the stampings unreadable. Also, a criminal will simply pick up his brass at the scene of a crime. If the firearms is a revolver, the microstamping requirement does not apply.)