Wikipedia
Rat-shot (or snake shot) is very small lead shot (typically #12 – 1.3 mm (0.05")) loaded in cartridges for use in rifled firearms as opposed to more traditional smoothbores such as shotguns. Rat-shot cartridges allow one to use a handgun or rifle as a low power shotgun for short-range use. Cartridges loaded with rat-shot are often called shotshells. The most common cartridges loaded with rat-shot are the .22 Long Rifle and certain pistol or revolver cartridges. Shotshell cartridges known to be available for hand guns include: .22 LR, .22 Magnum, .38 Special, 9×19mm Luger, .40 Smith & Wesson, .44 Special, .45 ACP and .45 Colt. The CCI .22 LR shotshell holds 1/15 ounce of #12 shot; the CCI .45 Colt shotshell holds 1/3 ounce of #9 shot. The smaller shot size typical of .22 shotshells is sometimes called "dust shot" and is used mainly by collectors of bird specimens.