Added 11-26-01 There had been some discusion on the Spudguns email list awhile back about a semi automatic air cannon, this idea had been with me for over a year now and I figured that I wouldn't ever get around to building it so i'll share it with you. Pardon the drawings I'm not very good with MS paint and a laptop, but they should give you an idea of what's going on. This is the gun in the "cocked" position, the grey piece is the bolt, it is a cross sectional view, so it would be a cylinder with another cylinder inside of it. The black bands around it are o-rings. The blue rectangles are the projectiles. The bolt is held back by some sort of spring that isn't pictured but is nessasry to bring the bolt to the rearward position to let gravity "load" another round. There is no air pressure present except behind the valve, which isn't pictured.

This is the valve in the firing position, all the red areas are air pressure. as soon as the valve is opened the air pressure pushes the bolt forward and moves the projectile into the barrel and blocks off the magazine. In the bolt there are machined ports in the face of it to allow air to pass thorough it and to the barrel. Once the air is exausted or shut off, the spring (remember it isn't pictured) returns the bolt to the rear and gravity lets another projectile load.

After all of that I hope that you understand how it works. If I were going to build it I would most likely use a solenoid valve on a large resovoir that is hooked up to either an air compressor or a regulated co2 tank. The solenoid would have to be triggered buy some sort of a timer, either a 555 in a one shot mode or a microprosessor (probably a PIC 16f84 was what I was going to use). The reason for the timer is so that you can shut off the air so the bolt will return to the rear position, that and once the projectile has left the barrel you may as well shut off the air anyways, that is why I was considdering using the microprosessor so I could put an infrared LED and collector in the end of the barrel so it could shut off the air as soon as it isn't needed There are inherrent disadvantages to this design, the air flow will be limited due to the "bolt actuating tube", also you have to fill the space that the bolt used to occupy in the closed position. If firing fast you better have an air compressor that will keep up. If you still have any questions don't hesitate to email me.