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Helpful and Interesting information

 SHOT GUN GAUGE

  • Gauge is a measure of shotgun caliber.
  • A smaller gauge number means a larger shotgun; a larger gauge number means a smaller shotgun.
  • Shotgun gauge is determined by taking a pound of lead and creating a set of lead balls exactly the diameter of the inside of the shotgun barrel.
  • If 12 lead balls weigh one pound, the shotgun is said to be 12 gauge. If 20 lead balls weigh one pound, the shotgun is said to be 20 gauge.

Here is a chart of some shotgun gauges which have been popular over time:

 

Shotgun Gauge

Bore Diameter

10

.775"

12

.729"

16

.662"

20

.615"

28

.550"

67

.410"

 

.410 caliber is an oddity among shotgun calibers in that it is commonly referred to by its diameter in inches instead of its gauge number.

 

Shot Size Table*

 

LEAD SHOT

Shot number
diameter (inches)
diameter (mm)
approx no. of pellets in 1 oz.
 
 
 
 
000
0.360
9.14
6.2
00 BUCK
0.330
8.38
8
0 BUCK
0.320
8.13
9
1 BUCK
0.300
7.62
11
2 BUCK
0.270
6.86
15
3 BUCK
0.250
6.35
19
4 BUCK
0.240
6.10
21 
BB
0.180
4.57
50
2
0.148
3.76
90
4
0.129
3.28
135
5
0.120
3.05
170
6
0.109
2.77
225
7.5
0.094
2.39
350
8
0.089
2.26
410
8.5
0.085
2.16
470
9
0.079
2.01
585
12
0.050
1.30
2300

 

STEEL SHOT

Shot number
diameter (inches)
diameter (mm)
approx no. of pellets in 1 oz.
F
0.22
5.59
40
T
0.20
5.08
53
BBB
0.19
4.83
61
BB
0.18
4.57
72
1
0.16
2.79
103
2
0.15
3.05
125
3
0.14
3.30
154
4
0.13
3.56
191
5
0.12
3.81
244
6
0.11
2.79
317
7
0.10
2.54
422
 
*source: Winchester Reloading Guide, NRA Firearms Fact Book

 

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                  Full Choke             Mod. Choke             Imp-Cyl Choke
                             (inches)            (inches)                 (inches)                  (inches)
 
      12                   0.729                0.694                       0.710                       0.720
      16                   0.667                0.639                       0.652                       0.660
      20                   0.617                0.592                       0.603                       0.610
      28                   0.550                0.528                       0.538                       0.545
      410                 0.410                0.393                       0.402                       0.406
Cylinder choke is exactly the same diameter as the bore diameter for the gauge.

 

This chart covers each of the chokes in the amount of constriction.

 

     Gauge            Bore                  Full Choke             Mod. Choke             Imp-Cyl Choke
                             (inches)            (inches)                 (inches)                  (inches)
 
      12                   0.729                0.035                        0.019                       0.009
      16                   0.667                0.028                        0.015                       0.007
      20                   0.617                0.025                        0.014                       0.007
      28                   0.550                0.022                        0.012                       0.005
      410                 0.410                0.017                       0.008                        0.004
Cylinder choke has no constriction and is the same as the bore diameter for the gauge.
 
-- From Shotgun Gunsmithing by Walker, a Gun Digest Book. P 157-158.
 

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