KB - choke constriction absolutely must be measured, especially absolutely on older guns that most likely have had their bores enlarged from shooting and maintenance. 13/1 tells you only that the bores would accept a 0.719" plug and would not accept a 0.729" plug at the time of proof inspection. Since the bores could vary by about ten thou even at time of manufacture, a muzzle measure could not possibly yield an accurate choke constriction estimation. You must get a bore diameter measurement just before the choke constriction starts and at the minimum diameter within the choke constriction.
Your current hot shooting may well be due to an open choke!
