Originally Posted by
W.R.Buchanan
The is such a thing in the Shotgun World known as a Bore Gage. It has a probe that has ball bearings that work against a wedge/cone with a Dial Indicator outside that pushes the balls outward.
You set the probe to a known diameter using a Ring Gage and then the bore is measured plus or minus that set dia.
Many serious Trap Shooters have and use these instruments and a good friend of mine makes one of the best.
There are several guys over at Trapshooters.com who make and sell them.
There are also several versions of the Bore Dia Chart below. and they are all slightly different. but usually +/- .005 on all the dimensions. IE: 10 ga runs .775-.780 12 ga. runs .729-.735 etc.
Randy