more like an overgrown shotgun cartridge.
Not 'overgrown' at all. A normal gauge cartridge. Admittedly rather bigger than a .410, but they just used the size for the job. It was an 8 gauge. Some wildfowling shotguns used a 4 gauge cartridge. I suppose most out there only think of the most common shotgun size – the 12 bore.
You can work out the diameter using the standard density for lead and 4/3 Pi r^3. Without checking, a 12 gauge is about 0.718". More like a cannon shell case than a bullet, I suppose.
Edited By not done it yet on 19/12/2017 07:33:51