Here's the deal with chokes: Chokes are measured by the percentage of
shot they put into a 30" circle at 40 yards. They aren't measured by
what is stamped on the barrel or by any particular choke dimensions in
thousandths of an inch. It's performance that's the measure.
Problem is that choke performance differs with each shell. I can
absolutely guarantee you that I can take a gun with a "modified" choke
and get improved cylinder (50% into a 30" circle at 40 yards),
modified (60%) and full (70+%) performance out of it by just changing
shells.
So it gets confusing because the standard is a moving target (please
note clever pun produced when fueled with only one cup of coffee).
Look at choke another way. The most open choke (cylinder bore) is no
choke at all. Just a tube. The tightest usual choke is full, excluding
some of the turkey chokes meant for special tight performance with a
very limited selection of shells. Cylinder bore throws about the same
pattern at 20 yards that full choke throws at 40 yards. Pretty close
anyway. All other normal chokes fit in between Cylinder Bore and Full,
or performance between 20 to 40 yards.
As a VERY rough rule of thumb, the following all produce exactly the
same pattern and pattern size at the particular distance mentioned:
Cylinder Bore 20 yards
Improved Cylinder 25 yards
Modified 30 yards
Improved Modified 35 yards
Full 40 yards
In order to sell more chokes and cater to a perceived need,
manufacturers have added some in between chokes like Skeet (in between
Cylinder Bore and Improved Cylinder) and Light Modified/skeet 2 (in
between Improved Cylinder and Modified).
Generally accepted (whatever that means in the unregulated,
unstandardized, un-everything world of choke designations) USA choke
dimensions in the 12 gauge are:
Cylinder Bore .000"
Skeet .005"
Improved Cylinder .010"
Light Modified (Sk 2) .015"
Modified .020"
Improved Modified .025"
Light Full .030"
Full .035"
X-full .040"
Note how cleverly there is a choke name for each and every .005"
constriction. This is meant to sell a lot of chokes and it does. If a
shooter thinks that a Light Modified gives him exactly the pattern he
wants at 27.5 yards (midway between IC and Mod) and if he thinks he
can actually judge the distance difference between 25 and 27.5 yards
on something flying through the air, well then there's someone who
will sell him what he wants. That makes everyone happy.
Remember, you have to add to all the above what I said earlier- I can
easily change the choke performance a full choke designation either
way by simply using a different shell. It really gets confusing.
I think that one of the reasons that some people pay so much attention
to choke is that it's something that they can actually do something
about and measure and buy. Learning to shoot properly is far harder
than simply buying another choke. That's not to say that proper choke
selection isn't of some importance. You don't want to shoot 27 yard
handicap trap with cylinder bore or go hunting woodcock in heavy cover
with full. To a champion clay shooter, choke matters because he'll win
or lose by only one bird out of 100, so everything counts.
But to the average guy, choke is highly over rated. There are really
only three constrictions that matter- close, medium and far. Those
would probably correspond to 20, 30 and 40 yards for most shotgunners.
For the average upland hunter, the improved cylinder and modified
combination has stood the test of time.
And now to confuse it further with the subgauge. Technically, all
shotgun chokes should be measured the same way- by the percentage of
pellets in the circle. BUT, reality has to enter the picture too.
Generally, you want to choke a subgauge gun tighter than a 12 gauge
gun to get equal performance. Let's say that the typical 12 gauge gun
load is 1-1/8 oz of pellets, while the typical 20 gauge uses 7/8 oz.
Let's also say that we are also using #8 shot. The 12 gauge 1-1/8 oz
load has 462 #8s, while the 20 gauge's 7/8 oz payload only has 359. So
a Modified choke 60% pattern with the 12 puts 277 pellets in the 30"
circle, while the same 60% modified pattern from the 20 puts only 215
in there. Both are the same modified chokes, but the performance sure
isn't the same.
To properly equate patterns, you must have the same pattern density in
actual pellet count, not in some percentage number. For the 7/8 oz 20
to get the same pellet count of 277 that the Modified 12 gauge 1-1/8
oz load has, the 7/8 oz 20 would have to get a 77% pattern. That's an
extra full choke to bring the little 20 equal to the 12's modified.
That's just something else to bear in mind, muddy the waters and cause
confusion.
Sometimes I think that the happiest people are those with fixed choke
guns who just fill 'em up and fire 'em.
from
http://www.shotgunreport.com/TechTec.../9-Jun-03.html 
favorite chokes ? 
yes indeed. My favorite.
