Choke Bore Sizes and Constrictions

The following is a generally agreed upon list of shotgun bore diameters for all gauges and a list of generally accepted choke constrictions (in thousands of an inch) for the various named choke constrictions.

Gauge 10 12 16 20 28 410
True Bore Dia. .775 .729 .662 .615 .550 .410
Cylinder Bore .000 .000 .000 .000 .000  .000 
Skeet I .005 .005 .004 .004  .003 .002 
Improved Cyl. .010 .009 .007 .006  .005  .004 
Skeet II .015 0.12 .010 .009  .007  .006 
Modified .020 0.19 .015 .014  .012  .008 
Improved Mod. .025 .025 .020 .019  .016  .011 
Full Choke .035 .035 .028 .025  .022  .015 
Extra Full .040  .040  .035  .027  .024  .021 

Shotgun choke is a constriction in the end (usually the last 3" or so) of a shotgun barrel which focuses the shot into a tighter stream--much like a nozzle on the end of a water hose, which does exactly the same thing for exactly the same reason. It is not the bore diameter of a shotgun barrel that determines the pattern percentages it shoots, nor the gauge. It is just that few thousandths of an inch of extra steel that is left in the (inside) end of the barrel when it is bored. The table above illustrates the constriction left inside a barrel and the choke size "assumed" by the manufacturer. 

To understand the choke of any shotgun, it needs to be patterned using a particular manufacturers shells.  Pattern percentages are normally taken at a distance of 40 yards. The number of pellets striking within a 30 inch circle, drawn so as to encompass as many pellet holes as possible, is counted and compared with the number of pellets contained in an identical unfired shell.   By counting pellets within the circle and computing what percentage of the total pellet count in a shell will yield the "choke type" for that shotgun using that shell can be assigned.  A different shell may give a different result

Type of Choke




Improved Cylinder

Skeet Light Modified




Extra Full


40% at 40 Yards

45% at 40 Yards

50% at 40 Yards

55% at 40 Yards

60% at 40 Yards

65% at 40 Yards

70% at 40 Yards

73% at 40 Yards

75+% at 40 Yards

Part of the confusion, no doubt, results from the fact that any given shotgun barrel is likely to pattern differently with various shells. Many anomalies occur between what is stamped on shotgun barrels and how they pattern with different loads. Trap loads, typically 1 1/8th ounce of hard #7 1/2 or #8 lead shot, often pattern tighter than 1 1/8th ounce "field" loads of softer and larger #4 or #6 lead shot. Steel and Tungsten shot typically patterns tighter than lead or bismuth shot. In both cases this is because the former is harder and deforms less than the latter on its trip down the barrel.

Probably the most useful way to pattern a shotgun is to test all the barrels you have at the distance you expect to break most targets (or kill most birds), with the load you intend to use. Then select for use the choke that gives the largest and most even patterns, without an excessive number of "holes" through which the target could escape. A variation would be to test a selection of appropriate loads through the barrel/choke combination that you intend to use, again looking for the largest and most even pattern without excessive holes. This system won't tell you what your pattern percentage is, but it will tell you which choke is most efficient for your purpose, and you don't have to do any counting.

The goal is to put more pellets into a target as well as being able to hit a fast moving target.  So the tighter your shot group, the more potential there is to not hit a moving target but it does increase the potential of putting a greater number of pellets in your target if you hit it (better odds of a "kill").  But the converse is also true, the wider your shot spread, the greater the probability that lower number of pellets will hit your target but you are more likely to actually hit it.  So it is always a trade off between hitting with a few pellets more often than hitting with many pellets less often; and a what distance.

So, depending on what type of shot you are shooting and also the type of game or target you are going for, you can fine tune your shotguns performance and find the happy medium of putting the most shot pellets in a target and having a spread big enough to hit the moving target that you are after; using the correct size shot pellet.

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