A firearm- Usually a handgun or revolver. This term came from the 30's mafia/gangster times. When continually fired, the barrel would get very warm- hence the name heater.

a gun, usually handgun

means gun. usually a 9mm or anyother pistol. mad popular by book "native son"

I believe in common usage that heater is a term now applied to all firearms, not only fully automatic firearms.

A hot fart. A fart where you can feel the warmth in your drawers long after the gas was passed.

Any gun.
Called like that cause it heats up the gun when you shoot it(quite obvious).

another word for a cigarette