US gun terminology. Switching your gun from semi-automatic (one bullet per trigger pull) to automatic (keeps firing until you let go or you run out of ammunition). Typically refers to the function in the increasingly rare pre-1986 Assault Weapons Ban automatics though older guns (i.e. pre-1960s import ban) guns. Because of the very unlikely chance of someone buying an older gun for anything beyond having fun with it, the automatic mode of fire is instead used for pure entertainment value, as an excellent way to expend incredibly large (and expensive) amounts of ammunition in a very short period of time.
Dan: How'd you end up going though 300 rounds in an hour?
Tom: I hit the fun switch on my Tommy gun.
Tom: I hit the fun switch on my Tommy gun.
by RedShocktrooper September 17, 2018