In this case, Congress did define multiple times these terms explicitly in the 1934 National Firearms Act, the 1969 Gun Control act, the 1986 Hughes amendment, etc.

ATF's job in this case should be enforcing the existing law, not writing new law with no congressional approval and then enforcing it themselves.

I agree that congress did give them some limited power to adjust to changing times, however I believe that has been abused in recent years to enact backdoor legislation via Presidential order without going through the expected legislative channels. That power should be revoked.