The nominal bore diameter is the diameter of the bore between the forcing cone and the choke in a shotgun barrel; the center section of the barrel if you will.  The choke is the constriction at the muzzle end of the barrels.

If a barrel has been worked to remove pitting these diameters will be larger than expected and indicate how much the barrels may have been worked.

Here are the nominal Bore Diameters for a few shotgun gauges still around today:

 

Gauge	Nominal Bore Diameter
4	1.052"
8	0.0835"
10	0.775"
12	0.729"
14	0.693"
16	0.662"
20	0.615"
24	0.579"
28	0.550"
32	0.526"
36	0.506"
.410	0.410"

Choke Dimensions
Choke dimensions are constant no matter what gauge or bore you are talking about. They are a fixed set of constrictions that are a numerical value below the nominal bore diameter.

In other words, you subtract the same amount from the nominal bore diameter to get a Full choke, for example, on a 12 gauge as you would on a 28 gauge.  For a 12 gauge it would be 0.729”- 0.040” = 0.689” exit diameter in the 12 gauge for a Full choke, and for the 28 gauge it would be 0.550”- 0.040”= 0.510” exit diameter for a Full choke on the 28 gauge.

Here are those dimensions and their names as commonly encountered:

 

Choke Name	Constriction Amount
Cylinder Bore	0.000"
Skeet 1	0.005"
Skeet 2	0.0075"
Improved Cylinder	0.010"
Light Modified	0.015"
Modified	0.020"
Improved Modified	0.030"
Light Full	0.035"
Full	0.040"
Extra Full	0.060"
Turkey Full	0.080"
Super Full	0.100"