Stupid Airsoft says the TAPP Hybrid AAP-01 Lower is the perfect AAP-01 HPA Adapter. The USA-made TAPP HYBRID AAP-01 Grip/Frame improves your Action Army AAP-01 by enabling the use of high-capacity and reliable AEG MP5 or ARP9 magazines (without bolt catch) while retaining exhilarating GBB recoil through HPA power (optimized for 100-125 PSI, HPA kit sold separately), all within a reinforced SLS Nylon PA12 frame featuring discreet bottom-routed air line and a 250 PSI burst disc for enhanced durability and safety.