Many shotguns have a constriction ("choke") at the muzzle that serves to reduce the spread of pellets as they leave the barrel. The amount of constriction varies between different models.
Constriction in 0.001 inches
Percent of Max
European Term
US Term
0
0%
cylinder bore
cylinder bore
5 to 10
10/40 = 25%
quarter choke
improved cylinder
20
20/40 = 50%
half choke
modified
30
30/40 = 75%
three quarter choke
improved modified
40
40/40 = 100%
full choke
full choke
length of choke in the barrel in inches =
= 5/8 * (constriction) / 10
For example, the length of full choke is 5/8 * 40/10 = 2.5 inches
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