"Ballistic fingerprinting" guns is a stalking horse: to make it work you'd have to make it illegal to modify a gun once fingerprinted, which basically means wiping out the entire aftermarket parts and repair business. If you can't repair grandpa's old Springfield -- well, that's one less unregistered gun on the loose, isn't it?
Also wiping out serious target shooting as a sport. I shoot benchrest and go through two new barrels each year. Most BR shooters consider a barrel to be shot out between 1500 and 2000 rounds.