or
zinc jackets 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. The traditional and still most common powerplant for BB guns is the spring-piston pump, usually patterned after a
lever-action rifle or a
pump-action shotgun. The lever-action rifle was the first type of BB gun, and still dominates the inexpensive youth BB gun market. The
Daisy Model 25, modeled after a pump-action shotgun with a trombone pump-action mechanism, dominated the low-price, higher-performance market for over 60 years (19141978). Lever-action models generally have very low velocities, around , a result of the weak springs used to keep cocking efforts low for use by youths. The Daisy Model 25 typically achieved the highest velocities of its day, ranging from . Multiple-pump pneumatic guns are also common. Many pneumatic pellet guns provide the ability to use BBs as a cheaper alternative to lead shot. Some of these guns have rifled barrels, but the slightly undersized BBs do not
swage in the barrel, so the rifling does not impart a significant spin. These are the types of guns that will benefit most from using precision lead BB shot. Pneumatic BB guns can also attain higher velocities than traditional spring-piston types. The last common type of power for BB guns is
compressed gas, most commonly the
Powerlet cartridges. The powerlet is a disposable metal
gas cylinder containing of compressed
carbon dioxide, with a self-contained valve to release the CO2 which expands to propel the BB. These are primarily used in BB
pistols, and are capable of rapid firing unlike spring-piston or pneumatic types. A typical CO2 BB pistol uses a spring-loaded magazine to feed BBs, and a
double-action trigger mechanism to chamber a BB and cock the
hammer. However some guns (either to stay true to the original gun or to make the trigger pull easier) do have a
single-action trigger. Either type of gun may also have
blowback action, where CO2 will push the slide back in addition to firing a BB. When firing, the hammer strikes an internal valve linked to the CO2 source, which releases a measured amount of CO2 gas to fire the BB; this also gives it realistic recoil and muzzle report features. Many CO2 BB guns are patterned after popular
firearms such as the
Colt M1911, and can be used for training as well as recreation. Some gas-powered BB guns use a larger source of gas, and provide
machine gun-like fire. These types, most notably the
Shooting Star Tommy Gun (originally known as the Feltman) are commonly found at
carnivals. The MacGlashan BB Gun was used to train
antiaircraft gunners in the
United States Army Air Corps and
United States Navy during
World War II. A popular commercial model was the Larc M-19, which used 1 pound (454 g) canisters of
Freon-12 refrigerant. These types have very simple operating mechanisms, based on a
venturi pump. The gas is released in a constant stream, and this is used to suck the BBs up into the barrel at rates as high as 3600 rounds per minute. ==Safety==