Shotgun
Choke Tubes
-- by Rob Ramsdale
--
At one time, all shotguns had no choking
on their barrels at all. Each shotgun had a barrel
that was the same diameter all of the way down the
barrel. Then several different people during the mid-
1800's discovered that if they constricted the barrel
some near the muzzle, they could get improved, tighter
patterns from their shotguns. There were several people
world-wide working on this at the same time so it's
hard to say who came up with the idea first . All
we know now is that it works very well.
Choke
Defined
Choke is a carefully measured constriction
of the bore of a shotgun at the muzzle, designed to
control the spread of the shot as it leaves the barrel.
Choke classifications are determined
by measuring the barrel with a bore micrometer. The
internal diameter is measured four inches from the
muzzle and again just at the muzzle. Subtracting gives
the amount of constriction in thousandths of an inch.
Below are descriptions of each shotgun
gauge and their constrictions for each common choke
designation. The pattern percentage is the percentage
of the pellets from a shell shot through that choke
restriction that hit inside a 30" circle at 40
yards distance.
12
Gauge
Constriction |
20
Gauge
Constriction |
28
Gauge
Constriction |
.410"
Bore
Constriction |
American
Designation |
English
Designation |
Pattern
Percentage |
.000"
-.001" |
.000"
- .001" |
.000" |
.000" |
cyl
|
true
cylinder |
40% |
.002"
- .006" |
.002"
- .004" |
.001"
- .004" |
.001"
- .002" |
skeet
|
impr
cylinder |
45% |
.007"
-.013" |
.005"
- .011" |
.005"
- .007" |
.003"
- .006" |
imp
cyl |
quarter
|
55% |
.014"
-.023" |
.012"
- .019" |
.008"
- .014" |
.007
- .012" |
mod
|
half
|
60% |
.024"
-.031" |
.020"
- .026" |
.015"
- .020" |
.013"
- .017 " |
imp
mod |
three
quarter |
65% |
.032"
-.040" |
.027"
- .033" |
.021"
- .027" |
.018"
- .021" |
full
|
full
|
70% |
.040"+
|
.034"+
|
.028"+ |
.022"+ |
xtra
full |
|
75% |
Of course, the dimensions used in
the above table are only rules of thumb. Bore measurements
by micrometer are useful to predict the pattern thrown
by a shotgun barrel, but they remain merely a prediction.
The only way to determine the actual pattern thrown
by a shotgun barrel is to shoot it at 40 yards, count
the percentage of pellets falling within a 30"
circle placed around the visual center of the pattern
(as in the right-hand column of the above chart),
then repeat it a few more times and take an average.
Modern turkey chokes definitely fall
into the "xtra full" range on this chart.
Some of these chokes when used with the right shell
and gun will throw 95% patterns at 40 yards.
|