III. Firearm Information by Type
     E. Shotguns
        1. Shotshells and Loads
           e. What's a Gauge Anyway?
	
by Scot E. Heath (scot@hpfistu.fc.hp.com)

                            SHOTGUN GAUGE AND CHOKE

The gauge number of a shotgun is the number of lead spheres the same
diameter of the bore it takes to equal 1 pound mass.  The density of lead
is 710 pounds/cubic foot.

The common shotgun bore diameters are then:

  10 gauge = .775 inches
  12 gauge = .729 inches
  16 gauge = .662 inches
  20 gauge = .615 inches
  28 gauge = .550 inches

These figures are all nominal and in fact, many competition guns have bores
larger than nominal to increase the velocity of standard ammunition.  One
barrel maker here in the United States makes a 12 gauge barrel with a bore
of .780 inches!

Choke in a shotgun is the amount of restriction from the bore to the
muzzle.  To determine choke, the bore as well as the muzzle inside diameter
should be measured due to variations in the diameter of the bore.  Briley,
a large manufacturer of specialized barrel liners, choke tubes, etc rates
their chokes as follows:

  .000 restriction - cylinder
  .005 restriction - skeet
  .010 restriction - improved cylinder
  .015 restriction - light modified
  .020 restriction - modified
  .025 restriction - improved modified
  .030 restriction - light full
  .035 restriction - full
  .040 restriction - extra full

The chokes most typically used in shooting American trap are the improved
modified or the full.  Many older guns have much more restriction than that
listed in this table.  As ammunition has improved over the years with
better wads and lower pressures, the need for such tight chokes has really
been eliminated for most shooting and in fact, is a detriment in many
cases.

-Scot