Rocket Airsoft AEG Full Gear Set - High Speed 24:1 Ratio Gears and Related Parts Padded back panel with mesh

Padded back panel with mesh reinforcement to maximize air flow

If they're bigger

Knight's Armament SR-16E3 Mod

The G&G Combat Machine 16 Raider L AEG Airsoft gun in Desert Tan color is a great M4 based rifle

The Redline Poppet shim kit gives you all the benefits of a low flow Poppet with the added advantage of shorter poppet dwell times and multiple possible air flow configurations

Rocket Airsoft AEG Full Gear Set - High Speed 24:1 Ratio Gears and Related Parts Padded back panel with meshRocket Airsoft Rocket Airsoft AEG Gear Set High Speed 24: 1 Ratio. CNC Steel made gears made to the same specs as SHS airsoft (OEM). Highest quality gear set for the money.