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.
Seems it wasn’t just “possession of an oil filter silencer”:
> Candelario sold and manufactured AR-15 style assault rifles in Maryland without a license. The ATF caught Candelario by using an anonymous man to purchase the firearms in a gas station parking lot. This agent purchased six rifles, as well as two fuel filter silencers.
Has this been tested with silencers specifically? Seems like it’s nasty both directions; if you usually run a silencer on your home defense gun, they’d get you for that. If you don’t, and you take the time to put one on, it undermines your self defense claim.