Under which Constitutional enumerated power does the federal government demand they have any authority to regulate firearms???
The “commerce clause”. The products move in interstate commerce, the few that haven’t all too easily can, and misuse thereof has significant impact on other interstate commerce. Ergo, the whole category falls under federal jurisdiction, whereby all interstate movement, or potential thereof, is regulated via FFLs.
That’s the reasoning. Don’t confuse my explaining it with me agreeing with it.