They make pretty decent silencers when affixed to the muzzle of a gun, and suddenly become an NFA item.

I know a guy with a diesel repair shop who bought a “suspicious” number of filters off Amazon at once. A couple suits showed up at his door asking about them. We totally live in a surveillance state, and Amazon feeds your purchase history into it.

It is issued in the form of enforcement action by the ATF. The recent SC ruling on regulatory agencies may change this some.

According the the NFA, anything you use as a suppressor, is a suppressor.

Be it an empty bottle, an oil filter, or a potato (yes, really).

Constructive intent is what the ATF claims (and wins in court).

the "fuel filter" thing with the right other stuff is constructive possession even if u dont build it.