#2 shot gives 17-2 = 15, meaning that the diameter of #2 shot is 15/100 or . B shot is , and sizes go up in increments for BB and BBB sizes. In metric measurement, it is easy to remember that #5 shot is 3 mm; each number up or down represents a 0.25 mm change in diameter, so e.g. #7 shot is 2.5 mm. Number 11 and number 12 lead shot also exists. Shot of these sizes is used in specialized shotshells designed to be fired at close range (less than four yards) for killing snakes, rats and similar-sized animals. Such shells are typically intended to be fired from handguns, particularly revolvers. This type of ammunition is produced by Federal and CCI, among others. Birdshot selection For hunting, shot size must be chosen not only for the range, but also for the game. The shot must reach the target with enough energy to penetrate to a depth sufficient to kill the game. Lead shot is still the best ballistic performer, but environmental restrictions on the use of lead, especially with waterfowl, require steel, bismuth, or tungsten composites. Steel, being significantly less dense than lead, requires larger shot sizes, but is a good choice when lead is not legal and cost is a consideration. It is argued that steel shot cannot safely be used in some older shotguns without causing damage to either the bore or to the choke due to the hardness of steel shot. However, the increased pressure in most steel cartridges is a far greater problem, causing more strain to the breech of the gun. Since tungsten is very hard, it must also be used with care in older guns. Tungsten shot is often alloyed with nickel and iron, softening the base metal. That alloy is approximately 1/3 denser than lead, but far more expensive. Bismuth shot falls between steel and tungsten shot in both density and cost. The rule of thumb in converting appropriate steel shot is to go up by two numbers when switching from lead. However, there are different views on dense patterns versus higher pellet energies. Buckshot Larger sizes of shot, large enough that they must be carefully packed into the shell rather than simply dumped or poured in, are called "buckshot" or just "buck". Buckshot is used for hunting medium to large game, as a tactical round for law enforcement and military