All comps work by using part of the gas to reduce recoil. The ones on pistols use ports to direct part of the gas upward like a rocket engine. This pushes the barrel down and counteracts part of the recoil. Sounds fine, but the drawbacks (aren't there always?) is that gas coming from the top of the gun is directed more toward the shooter than gas excaping from the front of the barrel, and is a lot noisier. There is also a flash from the ports which will probably not bother the shooter in daylight but can be a problem in low light conditions.
Jim
