What they don't know is there is a gun in the paint can

from Macklemore and Ryan Lewis (Ft. Ab-Soul & Macklemore) – Jimmy Iovine Lyrics on Rap Genius

Meaning

He’s meaning that he’s carrying a weapon the threaten Jimmy to sign him if he cant get it officially.

Again using paint cans as a metaphor for his music, he is saying there’s deep pain and desperation concealed in his music/paint can.

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