Brownells Brownells Brownells 75th Anniversary - A Shooting Heritage

Bore Diameters

Bore diameters vary from manufacturer to manufacturer, but the Small Arms & Ammunition Manufacturers Institute (SAAMI) does issue a recommended standard dimension for each gauge. These and the amount of choke constriction per designation is as follows:

This chart covers internal diameters
Gauge
Bore
(inch)
Full Choke
(inch)
Modified Choke
(inch)
Imp-Cyl Choke
(inch)
12
.729
.694
.710
.720
16
.667
.639
.652
.660
20
.617
.592
.603
.610
28
.550
.528
.538
.545
.410
.410
.393
.402
.406
Cylinder choke would of course be exactly the same as the bore diameter per gauge.

 

This chart covers each of the chokes in the amount of constriction
Gauge
Bore
(inch)
Full Choke
(inch)
Modified Choke
(inch)
Imp-Cyl Choke
(inch)
12
.729
.035
.019
.009
16
.667
.028
.015
.007
20
.617
.025
.014
.007
28
.550
.022
.012
.005
.410
.410
.017
.008
.004
Cylinder choke has no constriction and is the same as the bore in diameter.

-- From Shotgun Gunsmithing by Walker, a Gun Digest Book. P 157-158.