A quick video from KWA HK showing the recoil force of the KSC M4A1 ERG. The ERG or the Electric Recoil Gun is the AEG offering from KWA/KSC that has the recoil feature (RM4 and AKR). This seen in the KWA/KSC ERG series and other airsoft brands that use the ERG system of the company.
