Welcome to the forum, Robert.

The smarter more eloquent guys will be along in a bit.  The 30-40 head spaces on the rim and not the shoulder.   Personally I would not sweat the headspace if your grandpa used it to good effect and if it is in good mechanical condition.  Do you have any fired ammo with the gun?  That will tell much about the chamber.  Barring that, clean the rifle up and put a round through it and see what you got.  Then post a pic here of the spent round.