"Some advices I heard from the streets, they put a .38 special bullets on .357 so that it will not be easily traced if the gun is licensed. As well as to the .45, they interchange barrels. "
Huh? What are you talking about? Are you suggesting that if you use deadly force in a justifiable incident, that you should flee the scene? And why would .38 Spcl bullets be harder to "trace" than .357 bullets? Either would show the same rifling marks. But then, the authorities would have to either already have a sample bullet fired from that particular gun (or have the gun itself) to do a match.
Regarding semi-autos, yes you can swap the barrels. While that would change the rifling marks on the bullet, it wouldn't change the marks on the spent casing (from the extractor, the firing pin, and the breech block).
But again, we're (hopefully) all law abiding citizens here.