The origin of bust a cap meaning to discharge a firearm has become a general threat of violence with the implication that a gun will be involved. To threaten to bust a cap in someone's ass is also not intended to be anatomically specific, as kick ass or whip your ass are not descriptive of specific actions against someone's posterior. Ass as in "your ass" "my ass" etc refers more to the whole of the physical person. Further appellations such Honky, nigga, nigger, white boy, bitch, etc. are context dependent and may not have racial or gender implications.
"I'm gonna bust a cap in yo ass" "I would never bust a cap in your ass" "Please don't bust a cap in my ass" etc