Green’s Dictionary of Slang

break-down n.²

[such weapons can be ‘broken’ between the barrel and the stock]

(US black) a shotgun.

1998		Austin American-Statesman (TX) 4 Oct. 9/4: Texas Prison Gang Slang [...] Breakdown: Shotgun.
2002		Ac Dragon’s World ‘Gang Sl.’ 🌐 Breakdown Shotgun.