I'm Talkin' To You King T.I.

I know you won't bust a gun

to be more specific, i believe to "bust" a gun is short for "bust it out" as in pull it out of wherever it was previously concealed, which implies you won't shoot but also won't even have a gun to take out
Busting a gun means shooting it, so TI is saying that his enemies are cowards and that they won't actually be able to shoot him when they see him.