It's so true what Rob said here. When I got my new type Steyr it had the small green port and was dumping huge amounts of air on the shot. With the standard Steyr muzzle break it was as dead as a doornail.
After fitting a larger diameter black port I wound the hammer right off and the gun used far less air (obvious through reduced noise and increased shot count). It also resulted in a tiny amount of gun movement on the shot as, presumably, the stripper had less air to channel against rifle motion.