I don't currently own a gun, but I respect the rights of those who choose to do so. As many have pointed out, criminals will be ingenious in their efforts to defeat "fingerprinting" of weapons.

In fact, if the criminal doesn't wish to steal a gun, deface the rifling, exchange the barrel, etc., they have other options.

Lead poisoning is a concern for police and military shooters--as well as range operators. So-called "frangible ammo" was developed in part to provide lead-free ammunition with the additional benefit that the bullets virtually destroy themselves on impact, reducing the risk of injury from richochet. The unintended consequence is that a smart sniper with the means to obtain frangible ammo can fire, say, a .223 cartridge at 4,000 fps (most standard loads shoot at 2,000-2,500 fps) with almost no concern for recovery of the spent shell--it would be reduced to little more than tiny copper/tin fragments.

Here's a link with some additional information on frangible ammunition.

I'm not trying to educate the bad guys or frighten the rest of us. Like most bloggers, I think that the issues need to be thought through carefully before jumping head first into solution space.