The KWA MP9 HPA Adapter by TAPP Airsoft is reviewed by Stupid Airsoft as a necessity for using the gas blowback SMG in cold weather, as it switches the power source from inconsistent green gas to stable, cost-effective High-Pressure Air (HPA). The adapter replaces the heavy gas magazines with a unit that feeds BBs from common airsoft shotgun shells, offering a lightweight alternative. Whilst demonstrating its reliable performance on both semi and full-auto fire, the reviewer notes the main drawback is the loss of the MP9's function to lock the bolt back when empty.