And there are the pneumatic guns. I could write volumes on these guns, & I probably will. Pneumatic or air cannons/guns rely on compressed gasses to propel the projectile downrainge. Air cannons have an advantage over combustion guns in that not only are they quieter, but generally more powerful, & efficient than combustion guns of the same physical size.   Air cannons utilize compressed air, air that wants to escape the vessel it's contained in. Ambient air pressure is approximately 14.7 lbs/in, but air cannons require between 30 and 50 psi to operate effectively. That means that something or someone has to compress the air. Whether it's machine driven, gas, electric, or human powered, an air compressor of some sort is required. It can be a bicycle pump, or one that you have in your garage. That added requirement of a compressor might dissueade some from building this type of a gun, but its' performance is well worth the invenstment in a $150 dollar air compressor, or pumping the gun up manually everytime.   All air cannons use a valve of one sort or another to control & release the compressed air. There are many different types of valves & different designs for each type. For a complete listing of them click [here]. The simplest & easiest to build is a straight thru [picture] gun using a ball valve to release the air. However compressed air does not strike the projectile with the same amount of force [kinetic energy] as the rapidly expanding gas of a combustion gun, if we release the valve by hand we just can't turn it fast enough. Luckily for us there's an easy solution it's the [Solenoid Vavle]. It's an electric valve that can open in a fraction of a second.   Solenoid valves however do have a drawback. They are limited by the diameter of the ends [picture] If you have for example a 3" pressure chamber, and a solenoid valve with a 3/4" opening the volume of air in the 3" chamber is obviously restricted. Guns with barrel sizes close to that of the solenoid valve won't notice that much of a performance drop. However with larger guns & accordingly larger barrels & the heavier projectiles they bring, performance can be hampered. After a certain point it doesn't matter how much pressure or volume that you have in the pressure chamber, if you don't have the flow rate to the projectile, no perfomance increases will be realized. The general consensus is that performance is hindered, however it depends on who you talk to as to how much of a loss the solenoids incur when compared to a [piston/diaphragm gun] of the same proportions. Solenoid valves are a great place to start & provide a simple & powerful gun. Once you've tried a single solenoid valve, you might try to get added power & performance by using [multiple solenoids in parallel]. But there's a point where you just can't do any more with solenoids; At that point we come to the diaphragm or piston guns.    Discover more on Diaphram Guns >.