I think the suggestion of a good side-by-side gunSmith is a good one( I bet when the gun is clean inside the action, other than maybe replacing the small spring that retracts the shot gun firing pin, that everything will work fine, and I think youve heard our suggestions about making a cast of the chamber and bore Its easy,and will give you your answers
good luck in the end youll be glad you have a Hollenbeck drilling !