Green’s Dictionary of Slang

cap n.2

[mid-19C+] (US) a bullet, a shot.

In phrases

bust a cap (v.) (also bust caps)

[mid-19C+] (US) to fire a bullet.

pop a cap (v.)

[1900s; 1950s+] to fire a weapon; to shoot someone.

snap a cap (v.)

[mid-19C–1920s] (US) to fire a shot.