to 2 (3.8 mm, 0.15 in) and then BB (4.6 mm, 0.18 in). For buckshot, the numbers start and end with 4, 3, 2, 1, 0 ("single-aught"), 00 ("double-aught"), 000 ("triple-aught"), and 0000 ("quadruple-aught"). A different informal distinction is that "bird shot" pellets are small enough that they can be measured into the cartridge by weight, and simply poured in, whereas "buckshot" pellets are so large they must be stacked inside the cartridge in a fixed geometric arrangement to fit. The diameter in hundredths of an inch of bird shot sizes from No. 9 to No. 1 can be obtained by subtracting the shot size from 17. Thus, No. 4 bird shot is 17 – 4 = 13 = in diameter. Different terminology is used outside the United States. In England and Australia, for example, 00 buckshot cartridges are commonly referred to as "S.G." (Swanshot gauge) cartridges. Pattern and choke Shot, small and round and delivered without spin, is ballistically inefficient. As the shot leaves the barrel it begins to disperse in the air. The resulting cloud of pellets is known as the shot pattern, or shotgun shot spread. The ideal pattern would be a circle with an even distribution of shot throughout, with a density sufficient to ensure enough pellets will intersect the target to achieve the desired result, such as a kill when hunting or a break when shooting clay targets. In reality the pattern is closer to a Gaussian, or normal distribution, with a higher density in the center that tapers off at the edges. Patterns are usually measured by firing at a diameter circle on a large sheet of paper placed at varying distances. The hits inside the circle are counted, and compared to the total number of pellets, and the density of the pattern inside the circle is examined. An "ideal" pattern would put nearly 100% of the pellets in the circle and would have no voids—any region where a target silhouette will fit and not cover 3 or more holes is considered a potential problem. A constriction in the end of the barrel known as the choke is used to tailor the pattern for different purposes. Chokes may either be formed as part of the barrel at the time of manufacture, by squeezing the end of the bore down over a mandrel, or by threading the barrel and screwing in an interchangeable choke tube.