Weeeelp...

The fired case mouth ID measures ~.315-320
The fired case mouth OD measures ~.345-350

The bore at the muzzle measures ~.305. The caliper I used is only accurate to about 0.005, so I assume that's a pretty solid bet on the bore still being .308. If I measured it accurately, which is kinda questionable IMO.

Hmmmm. The OD of the fired case neck is pretty close to what .32 WS ought to be. I find it to be extremely difficult to measure bores and chokes accurately with a caliper, so I reckon I need to dig up some buckshot and slug it like you said.