Quote History Quoted:
This is another good reason for a trip to the gunsmith. Choking is surprisingly inconsistent in modern guns, and uniformly inconsistent in older guns
A gauge of the sort sold by Brownells will do fine in giving you the diameter of your muzzle and the corresponding theoretical choke, but that's only half the picture. Determining your actual/effective choke requires gauging the bore and finding out the relationship between your choke diameter and your bore diameter - frequently, neither will be what they're "supposed" to be.
For example, nominal bore diameter for 12ga is .729" with .020" constriction for a modified choke. So, you measure your muzzle at .709 and assume you have a barrel choked modified. However, if the actual bore diameter is, say, .740, now you've got a barrel that's actually choked a little tighter than full.
Did I mention that bore diameter gauges are expensive?
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Take it out and pattern it at 40 yards. That will tell you what you need to know about the guns choke with modern ammo.
In the old days mfg's had their designs and processes for choking a gun to get the right pattern at 40 yards. Choking was an art more than just ratio of muzzle to bore. When that gun was made ammo was loaded with card wads not full length plastic cup wads of today. It will pattern differently than marked, likely tighter iirc. My old 1930 ish fox ae patterns imp mod x imp mod with modern ammo. I think it was mod x mod as ordered by my grandfather.
Article about chokes and patterns here:
Patterning a shotgun
From the article there are a couple ways to classify choke: 70% in a 30" circle at some yardage, or percentage of shot in a 30" circle at 40 yards.
Idealisticly we are looking for shot and choke combinations that produce the following results when patterning.
Extra full choke delivers a 30 inch, 70% pattern at 45 yards.
Full choke delivers a 30 inch, 70% pattern at 40 yards.
Modified choke delivers a 30 inch, 70% pattern at 35 yards.
Improved cylinder choke delivers a 30 inch, 70% pattern at 30 yards.
Cylinder bore delivers a 30 inch, 70% pattern at 25 yards.
Or:
Choke is roughly determined for all shotguns by the amount or percentage of shot it delivers within a 30" circle at 40 yards.
Cylinder bore will deliver 40 percent of its shot load within a 30 inch circle at 40 yards.
Improved cylinder choke will deliver 50 percent of its shot load within a 30 inch circle at 40 yards.
Modified choke will deliver 60 percent of its shot load within a 30 inch circle at 40 yards.
Full choke will deliver 70 percent of its shot load within a 30 inch circle at 40 yards.