A BB gun is a type of air gun designed to shoot metallic spherical projectiles called BBs (not to be confused with similar-looking bearing balls), which are approximately the same size as BB-size lead birdshot used in shotguns ( in diameter). Modern BB guns usually have a smoothbore barrel with a caliber, and use steel balls that measure in diameter and in weight, usually zinc- or copper-plated for corrosion resistance. Some manufacturers still make the slightly larger traditional lead balls that weigh around , which are generally intended for use in rifled barrels (because lead is more malleable and exerts less wear on riflings).
The term "BB gun" is frequently used incorrectly to describe airsoft guns, which shoot plastic pellets (also often referred to as "BBs") that are larger (usually in diameter) but much less dense than metal BBs, and have significantly lower ballistic performance. The term is also sometimes used to describe a pellet gun, which shoots diabolo-shaped (not spherical) lead projectiles at higher power and velocity. Although some BB guns can also shoot pellets, the reverse situation is not true: steel BB balls have greater stiffness and are not meant to be shot from pellet guns, whose barrels are typically rifled and thus can get stuck (similar to a squib load in firearms) and lead to a damage or mechanical failure within the pellet gun.
History
The term BB originated from the nomenclature of the size of lead shots used in a smoothbore shotgun. Size "BB" shots were nominally , but tended to vary considerably in size because of the loose tolerances in shotshells. The largest shotgun pellet commonly used was named 00 or double ought and was used for hunting deer and thus called buckshot, while the smaller BB-sized shot was typically used to shoot small/medium-sized game birds and called birdshot.
In 1886, the Markham Air Rifle Company in Plymouth, Michigan produced the first wooden-construct spring-piston air rifle design as a youth training gun, and used the BB-size birdshot as the chosen ammunition. Two years later, the neighbouring Plymouth Air Rifle Company (later renamed Daisy Manufacturing Company in 1895) introduced the first full-metal airgun that also fired BB shots – the Daisy BB Gun, which became a very popular household name because of successful marketing. Around 1900, Daisy changed their BB-size bore diameter to , and began to market precision-made lead shot specifically for their BB guns. They called these "round shots", but the BB name was already well established, and consequently most users continued calling their guns BB guns, and the projectiles as BB shots or just BBs.
Subsequently, the term BB became generic, and is used loosely referring to any small spherical projectiles of various calibers and materials. This includes bearing balls often utilized by anti-personnel mines, .177 caliber lead/steel shots used by air guns, plastic round balls (such as the pellets used by airsoft guns), small marbles and many others. It has become ubiquitous to refer to any steel ball, such as a BB, as a "ball bearing". However, BBs should not be confused with a ball bearing, which is a mechanical component using small internal rolling balls to reduce friction between moving parts of machines.
Operation
BB guns can use any of the operating mechanisms used for air guns. However, because the projectile inherently has limited accuracy and short effective range, only the simpler and less expensive mechanisms are generally used for guns designed to fire only BBs.
Because the strength of the steel BB does not allow it to be swaged with the low propelling force used to accelerate it through the barrel, BBs are slightly smaller () than the internal diameter of the barrel (). This limits accuracy because little spin is imparted on the BB. It also limits range, because some of the pressurized gas used to accelerate the BB leaks around it and reduces the overall efficiency. Since a BB will easily roll unhindered down the barrel, it is common to find guns that use a magnet in the loading mechanism to hold the BB at the rear of the barrel until it is fired.