There is only one part that the law applies to for serial numbers- the reciever. By law, the receiver IS the gun. Many milsurp rifles have a "forced match" of SN on bolt and reciever- the number on the bolt removed, and a new one put on with an electric engraver. The purpose is to keep a headspaced bolt together with the correct rifle.
Do NOT change, obliterate, fiddle with, remove or modify the serial number on the reciever. To do so is a major, big, hairy Federal felony. Have been to Leavenworth- weather sucks, food is worse.
On any other part- help yerself.