BB guns and crossbows are two different things … right? Well, not if you're German weapons-tinkerer Patrick Priebe. Drawing on the design of his previously-made DIY airsoft rifle, he has now created a crossbow that shoots BBs.
Here's how the thing works …
Mounted in the middle of the aluminum-bodied device is a standard medical syringe. The end of that syringe (where the needle would go) makes an airtight seal with an opening in a block of wood. That opening is actually one end of a horizontal tunnel that has been drilled through the wood, with the other end opening into a brass tube – that tube is essentially a gun barrel.
Perhaps you can sort of start to see where things are going now. The idea is that when the syringe delivers a blast of air into the tunnel, a BB already sitting in that tunnel will be rapidly forced out through the tube.
The BBs are delivered into the wooden block through a second vertical tunnel, that joins up with the main horizontal one from the top. A weak magnet holds the bottom BB in place (with the others stacked above it), and the BB-loading opening gets plugged once the loading process is complete.
To shoot the crossbow, the steel spring bow is first pulled back and latched in place, drawing the syringe's plunger out in the process. Pulling the trigger releases the latch, allowing the bow to snap forward and forcefully push the plunger back in. A blast of air is delivered, and the bottom BB is shot out.
A green aiming laser helps users decide where that BB is going to go.
Priebe demonstrates the airsoft BB crossbow in the video below, and describes more about how it works.
Source: Laser Gadgets by Patrick Priebe