Rhyme about handguns
Gangster rappers of the 90s were the center of controversy for their supposedly positive portrayal of gun violence in their songs. Heems is being ironic, because the bottom line is that rapping about handguns is in no way violent or a bad thing.
Also a reference to the previous line about Jeff Mangum, whose last name is close to magnum, a handgun.
While not trying to get into an argument about whether guns cause violence, I would say Heems' sarcasm is more in the “It’s fun to do bad things” part of the line. Heems is all about that positive energy, not rapping about violence for the sake of violence.
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