You’ll get one of the best explanations I’ve read on what shotgun chokes do here.
Chokes today range from the most open, called “Cylinder,” to varying degrees of ultra-tight Extra-Full. Let’s take a look at the most common you’ll encounter for the most popular shotgun gauge, the 12-gauge.
A shotgun barrel with Cylinder choke actually has no constriction at all. Its diameter is the same as the barrel’s internal dimension. This choke throws the widest pattern, so it is considered the most open choke. Cylinder is good for close shots on clay targets and game birds, and is also the most popular choice for those who keep a shotgun for self-defense and use buckshot.
The Improved Cylinder choke is the first level of constriction up from Cylinder. It is 0.010-inch narrower in diameter than the barrel diameter. This provides just a little constriction to the shot column as it exits the barrel, enabling that group of shot to maintain a nice, wide pattern for fast flushing game birds at distances to about 30 yards. It is also a popular choice for skeet targets and sporting clays targets thrown at those same distances.
A Modified choke, the next tightest, will provide a bore reduction of 0.020-inch. It makes a good choice for an all-around shotgun choke. It works well on clays from 30 to 45 yards or so and can handle most of the flushing game birds. I’ve used a Modified choke for waterfowl, pheasants, and even turkeys. In fact, for years it was the only choke I owned, until I was educated about the shotguns with interchangeable chokes.
The Full choke was, for decades, the waterfowl hunter’s best friend. With a bore reduction of just about 0.035-inch, this tight choke allowed a shotgunner to hit geese and ducks at the 40-yard mark and beyond. The tradeoff with a choke this tight is that it’s usually too tight for close-in targets, those closer than 25 yards. You’ll either miss because the pattern hasn’t widened enough to hit a moving target, or you’ll absolutely annihilate a target with an almost literal fistful of lead. Such a hit is fine on a clay target, but if you’re a hunter looking for a pheasant dinner, you could be picking shot out of a pile of shredded meat.