[T. O'Heir]

Not seeing the pictures and Facebook says the page isn't available.
Slug the barrel. That'll give you the calibre. It won't tell you what cartridge though. That'll require Cerrosafe(Brownell's) and doing a chamber cast. Easy thing to do, but if you have any doubts, take it to a smithy.
From what the other posts say it could be any calibre or cartridge though. What your pop said is probably true, except there are more 8mm cartridges than just 8mm Mauser. It could be a .318" barrel or a rimmed 8mm. Slugging the barrel will tell you the first for sure. A chamber cast will tell you what the case looks like.