Most shooting clubs restrict shot sizes to number 7-1/2 or smaller. This number indicates the size of the individual pellets in the shell. The diameter of a number 7-1/2 pellet is 2.41 millimeters. Smaller pellets include number 8 (2.29mm), numer 8-1/2 (2.16mm), number 9 (2.03mm) and number 12 (1.27mm). Since these pellets are being shot up into the air, larger, heavier pellets are restricted because their inertia carries them farther, and they may be harmful to people standing hundreds of yards away.
Number 7-1/2 and number 8 shot work the best on clay pigeons. Shots in this sport are made at ranges between 16 and 27 yards, so a decent spread with a lot of small pellets is the most important characteristic of a useful load. Number 7-1/2 shot should be used for longer-distance shots because it is the heaviest allowed at most clubs and will carry farther.
Most shotguns support both 2-3/4 inch shells and 3-inch magnum shells. Magnum shells are unnecessary for shooting clay pigeons. In fact, most clubs restrict shot weight to 1 ounce. A standard 2-3/4 inch 12-gauge shell holds about 1-1/8 ounces of shot, so shooting clays requires a small, low-powered shell.
All shotgun gauges can be used to shoot clays. The most popular are 12, 16, 20 and 28 gauge. The 12 gauge has the largest-diameter bore and the highest capacity for shot, but it also has the highest felt recoil which can make shooting less comfortable than with smaller gauges. The .410-bore shotgun has an incredibly small diameter bore. It is impractical for hunting and was invented for skeet and clay shooting as a handicap for expert shooters.