the original black powder used in shotgun shells, so very little space is actually taken by powder; shotguns use small quantities of double base powders, equivalent to quick-burning pistol powders, with up to 50% nitroglycerin. After the powder comes the wadding or wad. The primary purpose of a wad is to prevent the shot and powder from mixing, and to provide a seal that prevents gas from blowing through the shot rather than propelling it. The wad design may also encompass a shock absorber and a cup that holds the shot together until it is out of the barrel. A modern wad consists of three parts, the powder wad, the cushion, and the shot cup, which may be separate pieces or be one part. The powder wad acts as the gas seal (known as obturation), and is placed firmly over the powder; it may be a paper or plastic part. The cushion comes next, and it is designed to compress under pressure, to act as a shock absorber and minimize the deformation of the shot; it also serves to take up as much space as is needed between the powder wad and the shot. Cushions are almost universally made of plastic with crumple zones, although for game shooting in areas grazed by farm stock or wildlife biodegradable fiber wads are often preferred. The shot cup is the last part of the shell, and it serves to hold the shot together as it moves down the barrel. Shot cups have slits on the sides so that they peel open after leaving the barrel, allowing the shot to continue on in flight undisturbed. Shot cups, where used, are also almost universally plastic. The shot fills the shot cup (which must be of the correct length to hold the desired quantity of shot), and the shotgun shell is then crimped, or rolled closed. Sizes Standard Shotgun shells are generally measured by "gauge", which is the weight, in fractions of a pound, of a pure lead round ball that is the same diameter as the internal diameter of the barrel; in Britain and some other locations outside the United States the term "bore" is used with the same meaning. This contrasts with rifles and handguns, which are almost always measured in "caliber", a measurement of the internal diameter of the barrel measured in millimeters or inches and, consequently, is approximately equal to the diameter