Originally Posted by arcanuck
Depending on how well you want it to work, the filling to use will depend on the sound absorbent material's NRC and what frequencies you want to remove. The suppressor's internal volume will be a major deciding factor in how much can be used and also in determining the volume's Fs (resonant frequency) and second high order Fs, the suppressor will also amplify these sounds and can defeat the purpose.
Wanker nerd. Lol.
Do what I do, get copper pipe foam insulation from a hardware store, cut into 1/2" slices, then take half of them and cut the thickness down to 1/4" or so. Alternate the slices, thick and thin, this creates foam baffles within the suppressor. Cut each to the right size to fit snugly (after expanded) into the tube, and use a barrel or a rod to press all into the outer shell of the suppressor. Literally what you want is vertical surfaces for the gas and the report to hit, isn't so much just open cell foam to absorb it, you need to have obstructions.