Take it to your local gunsmith and make sure it has not been back bored. You really do not want to shot slugs from an Older(?) shot gun if it has not been checked out. Ask about the choke, if it's tighter than Modifide I'd be cautious about that also(don't want to have more pressure that the barrel will take) Hope this points you in the right direction.

Misplaced Buckeye