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."