G&P High Speed Tactical Magwell for Airsoft M4 M16 Series Airsoft AEG - OD Green

"For only $18 I think every player who runs an M4 should have this. You can very honestly change mags with your eyes closed, meaning, you dont have to take your eyes off your target to insert a magazine. Initialization is incredibly easy and for only $18, this is honestly the most useful external upgrade you can buy.