Here is a list of EBBR Systems:
http://airsoftcanada.com/showthread.php?t=173245
The Bolt M4 has a good amount of kick and is more is is capable of upgrading than the KWA ERG.
The KWA ERG, GBLS DAS M4A1, TOP M4A1 EBBR Shell Eject and the TM EBBRs feature a bolt lock functionality.
TM EBBRS and the GBLS DAS M4A1 are expensive.
The G&P Free Float Recoil System is nothing special.
The TM Recoil Shock and the KWA ERG require special magazine to utilize the bolt lock/bolt release feature.
The BO-Dynamics/Lonex BAW-R kicks a lot like a GBBR but it does not feature a bolt lock/release.