Pop A Cap In Someone's Ass Meaning

(slang, idiomatic, US) To shoot someone with a gun.

Example: 2007, San Francisco Chronicle - 'Cover me, I'm going to Starbucks'
  The older teen was coming from "Tilt," the second floor arcade, when the shooter confronted him, threatened to "pop a cap in (his) ass," and, after the victim assumed what eyewitnesses say was a fighting stance, apparently did.
2004, R. Micharl Sullivan - The Dead of Winter - Page 130
  I'll give you thirty seconds to get out of here before I pop a cap in your ass. C'mon, move it. I'm counting now, one, two, three—better get your ass movin', four..."
  Carter started running, expecting to hear the shot and feel the rip of a bullet in his back.
1994, Pulp Fiction (on Wikiquote)
  I been sayin' that shit for years. And if you heard it, that meant your ass. I never gave much thought to what it meant. I just thought it was some cold-blooded shit to say to a motherfucker before I popped a cap in his ass. But I saw some shit this mornin' made me think twice.