Take the seamless metal pipe with an inner diameter of 1.5-2 cm and a length of 150-200 cm is best to use aluminum or duralumin pipe. The pipe should be straight without bends and deformations along the length.
If you wish to obtain more powerful weapons, make the tube of two or three sections. Such a collapsible tube you can in a few minutes to disassemble and stow in a travel bag. Mount sections are produced by means of connecting tube is slightly larger diameter. The connecting tube is put in the joint parts and is tightened by means of clamps. The tool clamp screws.
From the syringe or plunger are manufactured mouthpiece. It can be rounded or rectangular shape. The purpose of a mouthpiece to protect your lips from the connection with the metal. You can make a mouthpiece from a plastic tube from a bottle of champagne. For this tube, cut the bottom and firmly drive her in the tube.
Cut the tube facing towards the target in stowed position close suitable sized plastic cover. Cap will not allow the contaminated inner surface of the tube.
As the shells you can use skewers made of bamboo and even paintballs. Here before you open up unlimited horizons for creativity. Boom length can be 10 to 30 cm tail end of the boom take a few thickened, slightly smaller in diameter than the inner diameter of the tube. This will create the necessary pressure when blowing the projectile out of the tube.