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...
You don't have to replace or tune anything. Guns have a long lifespan, and normal wear will change the signature over time. These are not fingerprints. What are you going to do test the guns every week, or just every time the owner cleans his gun?