I wonder if he rifled the barrel for a more modern artillery shell that may be on the drawing board, something that goes boom where it lands?
You used to be able to buy plans for a .22 cal small scale replica of a Gatling gun which any reasonably competent machinist could build.
Fasten said Gatling to solid base, add simple chain drive in place of crank, connect chain to starter motor w/12 volt battery and, viola!, a minigun.
Remember, when you test fire, black helicopters will show up and follow you home.