General Discussion

Sounds suppressors are controlled items and generally cannot be mailed except to/from a licensed dealer/manufacturer or the registered owner (for repair). The prcess of selling one to someone else requires a lot of paperwork and time.

The part of the gun with a serial number is legally "the gun." All the rest of the parts are just parts and legal to buy, sell, mail.

You can't easily convert a manual action rifle into a semi-automatic, though semi-automatics are easy and cheapish ot get. Did mean a semi-automatic into an automatic? Parts that do that are illegal, or subject to heavy regulation. BUT.... there is an easily available part for AR-15's that turn them in to autpomatic weapons in all but name. They TECHNICALLY do not meet the definition of a machine gun... but they are a machine gun for all practical purposes.