The typical chokes are:
Cylinder = 0 constriction (ie, the muzzle diameter is the same as the barrel diameter).
Improved Cylinder = .010" constriction
Modified = .020" constriction
Improved Modified = .030" constriction
Full = .040" constriction
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True
The more constriction you have the smaller the muzzle diameter and the tighter the shot cloud.
Not exactly. In order to understand points of contstriction as listed above, you have to know that the constriction isn't determined by the choke diameter. It's determined by the DIFFERENCE between the INSIDE bore diameter and the constriction at the muzzle.
Obviously, as the choke diameter gets smaller, the bigger the difference, and the tighter the pattern.
If that difference is 40 pts. of constriction or .040, the choke is full, and so on.
Two barrels with the exact same constriction at the muzzle can have different patterns, simply because of variations in the inside bore diameter.
Also, know the true test is patterning the shotgun at 40 yds. in a 30 in. circle. The % of shot in that 30" circle will tell you what your pattern is with a given load.
TIP: Shoot first, then draw the circle
Expect smaller shot to have bigger patterns than larger.
Expect chokes set up for lead shot to pattern much tighter with steel shot.
Know that the figures listed are for 12ga. For determining 20 ga. chokes, we're working with fewer points of constriction having the same effect on patterns.
The gunstore I frequent has a gage that can be inserted into the bore to determine it's inside diameter. Then by measuring the choke at the muzzle, and subtracting that from the inside bore diameter, I have a good idea what to expect from a barrel. Still--patterning on paper as described is how to really know your shotgun.
Gauge is determined by the number of lead balls the diameter of the bore that equal about a lb.
Except for .410, which is a reference to bore diameter.
It's true that the barrel length has nothing to do with tightness of pattern, except that longer barrels tend to be choked modified and full. Longer barrels for long range waterfowl hunting, for example.
Open pattern chokes tend to be shorter. Quail and grouse hunting at closer range, for example, need to be faster handling than a barrel used for pass shooting geese.
With modern screw- in, interchangeable choke systems, barrels tend to be shorter than fixed choke barrels.