If you are new to shooting and do not understand shotgun choke tubes, you are not alone. A very large percentage of shooters that have shot shotguns for years do not understand shotgun chokes either. Simply stated,
choke tubes were designed to control pattern diameters at different distances or ranges. Shotgun ranges are described in yards. Pattern is just the grouping of the pellets at a given range or yardage. The pattern is the grouping of shot delivered to a circular pattern. In other words, for a choke to be effective, it has to have a certain number of pellets in a given area.
Ok - so given a range/yards and a circle target diameter, a more open choke (say cylinder or skeet) provides a less dense pattern (fewer shots) than a tighter choke tube (say full choke). That means that a shooter can select a choke the will provide a know shot density which is compatible with the target at a give range./yards. A very handy adjustment when you are shooting ducks at 15 yards today and 55 yards tomorrow.
How does choke work? A shotgun barrel is basically a long pipe with the chamber and shell at one end. At the other end is the opening where the shot stream will exit. The exit end is where the choke tubes are located. Within limit the tighter or smaller you make the choke tube hole, the tighter the pattern at yardage.
There are a number of choke tube manufacturers, like Browning, Beretta, Benelli and Carlsons,
and a great deal of science and experimentation goes into making a good shotgun choke. Geometry such as barrel length, barrel bore and taper and choke restriction are the basics to choke tube design. But ammunition as well as
choke material, geometry of the choke and even the choke's finish will effective the performance of any given choke.