Dawed, the issue I see is that in times past someone damaged the screw slots by using a very poor fitting screw driver. This is often a clue that other sub-standard work was performed inside. Hopefully, the only thing that happened was Fumb leFingers satisfied his curiosity. My formula has proven reasonably reliable to get you in the ball park.
Per 2-p above, plEEEEase don't fire a 3" shell in it. Take it to a qualified smith and have it inspected as suitable for firing. And, for pete's sake don't take it behind the barn, strap it to a tire, and pull the trigger on a modern high pressure load with a good long string (method sometimes refered to as "Firestone Proof"). Chamber length is far less important than load pressure - you want loads of KNOWN low pressure. Don't shoot anything until it gets a clean bill of health from a good smith.
2-p thinks it is over $150. Anyone else?
DDA
