Never seen bullet casting or reloading machines, has he?
Don't tell him about the 12ga conversion sleeves that allow you to shoot .22 and .38sp through a shotgun, or even a flare gun.
Or "Navy" Garands, that had a plug shoved into the chamber shoulder that permanently converted them from .30-06 to 7.62x51.
Adapting a .355, .400, .429 or .451 diameter firearm bore and chamber for a new cartridge is just a matter of a drop-in barrel in many cases. .400 Cor-bon? All those Glock 23's that were sold in .40 but are shooting 9mm now thanks to aftermarket barrels?