First time I heard of a oil filter being used as a silencer was in an old movie.
No threading was required, the bad guy just shoved the muzzle of a small caliber revolver into the hole so it wedged in place.
I'd never thought this would work till I saw a video demonstration of the thread adapter not long ago.
A small caliber revolver can be muffled quite effectively by a throw pillow, or practically anything thick and soft that you can wrap around the frame and muzzle.
The item should be damp or it can smolder and catch fire later on.
In England horse breeders can buy silenced small bore shotguns. The barrel has ports drilled for most of its length and theres a barrel jacket. Apparently this work well for killing vermin without spooking horses.