It's quite easy with some guns to swap out the barrel. It's also possible to do things like lapping the barrel or using different ammo to change the "fingerprint" of the gun. Or one could use a sabot round and leave no telltale barrel marks on the bullet at all!
There are several posts addressing the technical aspects of "fingerprinting" guns on this thread
A marginally adept hobbyist can turn a .308 SKS into a .223 by the simply expedient of using a plastic or wooden sabot. Remington tried it a few years ago, and while you had a 4,000 ft/sec .223, accuracy fell off dramatically.
A friend of mine once tried loading a .223 sabot in a hot .300 Winchester Magnum. He claims that it didn't work because the centrifical force was so massive that the bullet was flying apart within 100 yards. There were no prints on the paper, and at 200 yards he could easily shoot a 2 inch group with normal loads...