To fire a gun (military slang from the days of percussion cap and ball firearms before the manufacture of individual cartidges consisting of a metal casing holding a bullet, gunpowder and a primer that fired when struck by a firing pin or hammer.)
This is not a new phrase as much of the Rap culture would like us to believe. Instead it is an old phrase to shoot someone. Reference to cap is a word for powder cap used in percussion guns popular in the old west. This phrase is used in the 1969 movie titled "True Grit" with John Wayne, Glen Campbell, Dennis Hopper and Robert Duvall.
Line by Ned Pepper (played by Robert Duvall) "I never busted a cap (bust a cap) on a woman or nobody under sixteen. But I'll do it."
By Minerva
Bust A Cap
Attested in "True Grit" by Charles Portis, 1968, but most likely much older.
The derivation is probably from the percussion "cap," a small metal cylinder open at one end with an amount of shock-sensitive explosive, usually fulminate of mercury, used to set off the powder charge in a muzzle-loading firearm.
The modern derivation, however, is probably from the "cap gun," a toy firearm using paper "caps" containing Armstrong's mixture or a similar substance to provide the small explosions.
"I have never busted a cap (bust a cap) on a woman or anybody much under sixteen years but I will do what I have to do." -- "Lucky" Ned Pepper
By Natalie
Bust A Cap
To discharge a handgun, usually a 9mm, in the direction of an enemy or a rival gang.
I found this reference to "bust a cap" in a 1951 Gene Autry novel. Gene Autry and the Golden Ladder Gang by W. H. Hutchinson. Whitman 2349
"I guess it is," said Pres as he regretfully fondled the stock of his rifle, "but I surelee hope that somebody gives me a little excuse to bust a cap in their direction."
-- Gene Autry and the Golden Ladder Gang 1951
A slang phrase referring to the posession and use to a certain individial of a firearm or any other object resembling a firearm that emits any sort of a projectile.