The YouTube video, posted on 10 May, says the glitch works in any version of Rockstar Games' 2006 title without mods or external tools.
The rifle is normally limited to the shooting-range minigame, but the exploit lets players carry it into free roam, where it can damage NPCs and even fire during cutscenes.
The find emerged from a six-month pacifist challenge and suggests cut or unfinished content, possibly removed to avoid controversy over shooting on a school campus after earlier Rockstar scrutiny.
Was Bully's cut rifle a victim of media fear, or just another buggy, unfinished game feature?
After recent data breaches, will we finally learn the true reasons Rockstar cuts controversial game content?
As AI now finds thousands of bugs, is the era of human-led exploit discovery in classic games over?