Those fingernail shells come out my pistol top
In this verse Lupe is comparing his writing-hand to a gun and his words to bullets. In this line he compared loss of fingernail to a cartridge leaving the chamber after the shot. Shell-Case is also often used term for cartridge. Earlier in this verse Lupe was using his hand to make a gun of it (and the other one as a silencer) with his thumb cocked. When you shoot a “mock gun” you thumbnail is coming out of the top of the index finger just like shell-case would come from a real gun.
Lupe compared his music to weapons before.
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