The PTS Centurion Arms CM4 ERG airsoft gun uses an ERG (Electric Recoil Gun) system that is based on the latest generation KWA AEG 3 system. The ERG simulates recoil via a proprietary Kinetic Feedback System. When used with the proprietary ERG magazine, that is switchable between 30 or 120 round capacity, it is capable of an electrical auto cut-off when the last round is fired. If you are willing to forgo the cut off, the ERG can still use any normal AEG magazine such as the PTS EPM for AEGs. A quick change spring system allows you to easily customize the fps/m/s of the gun to your specific needs. Included as standard equipment for the CM4 ERG, are PTS’ highly popular proprietary design Enhanced Polymer Grip, Enhanced Back-Up Sight, Enhanced Polymer Magazine and Enhanced Polymer Stock.