Everything

Silencers make a handy addition to many types of guns, even though they do not entirely silence a weapon. They do muffle the sound of gunshot and marginally slow down the speed of the bullet to just under the speed that would cause a break in the sonic barrier (with its resulting “boom!”). Silencers do not work well, if at all, on revolvers. The structure of the revolver makes covering the gap between the gun barrel and the gas releasing cylinders impossible. Multiple shots also wear down the effect of a silencer, which works best on a single-shot sniper’s or executioner’s weapon. An extremely talented gunsmith could make a nearly soundless weapon, but the result would be a permanently altered and highly illegal weapon.

Basic manuals on guns and ammunition, either in book form or on the internet, can provide useful information for roleplayers otherwise unfamiliar with firearms.