PTS Centurion Arms CM4 C4-10 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, which 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’s highly popular proprietary design Enhanced Polymer Grip, Enhanced Back-Up Sight, Enhanced Polymer Magazine, and Enhanced Polymer Stock.