The BB gun powerplant - how it works
by B.B. Pelletier
You probably know about the three common airgun powerplants - spring-piston, pneumatic and CO2. Actually, there are several types of pneumatics, including multi-pumps such as the Sheridan Silver Streak, single-stroke pneumatics such as the Beeman P3 and precharged pneumatics such as the AR6 by Evanix. Today, we'll look at a different type of spring gun that uses both the power of the spring directly, such as a catapult gun, and also functions as a spring-piston airgun.
What I'm about to show you is a very common type of mechanism, but it's not universal. There are some BB guns that have different powerplants, such as the Crosman 350, 3500 and M1 Carbine. But this mechanism is what most collectors think of when they refer to a BB gun powerplant. It's the same one found in the Red Ryder.
By combining the catapult and the spring-piston, designers got a lot of power from a relatively weak mainspring. The catapult works initially to get the BB off its seat and moving, then a blast of compressed air from the spring-piston kicks the BB in the tail to boost it to maximum velocity.
This is a BB gun mechanism in the cocked state. The air tube moves with the piston as it goes forward. It shoves the BB to an initial velocity before the compressed air rushes through the tube to boost it.
For this type of mechanism to work well, the BB needs to be held fast at the breech end of the barrel, awaiting a shove from the air tube. Initially, manufacturers relied on a slight constriction in barrel diameter to hold the BB. That worked as long as BBs were made of pure lead, but did not work when they switched to steel in the 1920s. A wire spring was then used for many decades to hold the BB in position. It also worked very well, and the spring could be easily pushed out of the way by the air tube when it passed, so it was a good solution for a long time.
When magnet technology improved in the 1960s, small magnets soon replaced the wire spring, and then nothing had to be pushed out of the way. Magnetic shot seats are the standard today.
Now the gun has fired. The air tube pushes the BB up to 80-100 f.p.s., while the air is being compressed by the piston seal. The compressed air then rushes through the air hole at the base of the air tube and up the hollow tube to get behind the now-moving BB. It boosts the BB up to its final speed.
This mechanism is one of the reasons I am an airgunner. When I look at the simple design and scant materials that are used to make a BB gun powerplant, I'm overwhelmed by the creativity of the human mind. It takes a lot more ingenuity to propel a steel BB this way than it does to propel a lead bullet using a chemical explosion!