ive seen this done first hand using a larger oil filter the size of my forearm from a building-sized generator
there was some machining done to open up the closed end to allow the bullet to exit and something done to the threaded end to make it fit a threaded pistol barrel. WD-40 was sprayed into the filter every 20 rounds or so and it distinctly made it quieter compared to using it without any WD-40. it stunk to high heaven though
