The only way of getting an accurate reading is with a bore mic. It's how many thousands constriction of the muzzle opening compared to the bore diameter. A lot of chokes are opened up latter from the muzzle end. This leaves a longer parallel at the end. A good smith can see this just by looking down the barrel, or with a mic it could be checked. The now standard 12ga bore is .729, but many are over bored up to around .735 to .740. That's why a mic is needed. I have a Parker top lever hammer gun that has .744 bores. I believe that back then a little over size damascus tubes weren't thrown away - they were finish bored from the breech end and the amount of constriction was left for the chokes they wanted. Have a good gun smith look at them. Good luck.
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Paul Harm
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