** The Protech Mk2.5 M4 HPA Kit V2 Engine is a low manintenance and more advanced than you typical AEG gearbox. This is a drop in kit that requires professional installation and additional equipment to function properly. You will need to have an AEG, HPA Regulator, and HPA tank to make the whole system work. All these are available her at www.airsoftmaster.com or at our walk-in store.
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