An automatic BB gun, also known as a pneumatic BB gun, differs from the traditional spring piston BB gun; a BB gun with a spring piston has a barrel that pivots and is manually operated through a lever that causes the piston to move. When the piston moves, the mainspring is compressed, and the trigger is used to release the compressed mainspring. The BB is then expelled. In contrast, an automatic BB gun is a gun that uses compressed air to expel the BB projectile. You can construct a single-stroke automatic BB gun at home in 20 minutes.
1. Drill a hole directly in the center of the 1/2-inch plastic cap using the 1/4 drill bit.
2. Prime the exterior of the 2-1/2-inch piece of 1.5-inch pipe and the interior of the 1-1/2 plastic adapter. Spread glue inside the adapter where it has been primed. Glue the exterior of the 2-1/2-inch piece of pipe; insert the 2-1/2-inch long piece of plastic pipe into the 1/2-inch plastic adapter.
3. Glue on the interior of the 1/2-inch cap. Place the cap on the exposed end of the 1-1/2-inch plastic pipe adapter. Squeeze the 2-1/2-inch plastic pipe section and the 1/2 plastic cap together tightly so that the seams meet.
4. Drill a hole at the ridge of the 1-1/2-inch plastic adapter using the 1/8-inch drill bit; make the hole so that it goes into the center of the connected plastic pieces, but not through it. Apply Teflon tape to the 1/8-inch plastic nipple and screw the nipple into the front of the blowgun.
5. Apply Teflon tape to the 3/8-inch plastic nipple and screw the nipple into the front part of the blowgun. Apply Teflon tape to the quick connect and insert into the back end of the blow gun.
6. Attach the blow gun to the plastic pipe construction where you drilled the hole, twisting the blowgun and attached components.
7. Sand the surface of the cap with the 1/4-inch hole. Slip the 24-inch brake line through the hole; be sure to push the line all the way through until the end of the line stops just before the hole inside the interior of the cap. Use JB weld to fix the line into position.
8. Load BB rounds into the back of the device and seal off with the threaded cap. Hook to your air compressor.