I am going to respectfully disagree with Fastjack...in part.
Every legally sold firearm would probably have ballistics registered, along with shell casing marks (firing pin and extractor indentations). However, a technically minded individual with a CnC machine or one of those desktop nano forges with the schematics for lets say a Ruger Thunderhawk would be able to produce that weapon and even put a fake serial number on the pistol. Of course, you still have to worry about your gunsmith getting busted and offering you up for a reduced sentence. Nothing is perfect some are simply better than others.
Ammunition can be handled in much the same manner allowing you to bypass micro stamping on the projectile and the casing...right up to the point where you get to the gun powder. Tagnents are already common place in explosives and I expect the same to be true for gun powder within the next ten years time. Not really sure how to get around this one.
The idea is that you want a clean firearm that you can toss after using. The longer you keep that weapon and use it, the more crimes it ties you too. Steal a registered firearm? Ties you to the original theft. The cops start back tracking and see if they can match a vehicle or picture of you to traffic cams, ATMs, or security cams in the area prior to, during and shortly after the theft.