The Sig Sauer 21rd ProForce P320 M17 GBB spare airsoft magazine by Sig Sauer is made from a heavyweight metal alloy, giving a hefty feel to the Sig M17 airsoft pistol. This green gas Sig Sauer magazine has a high-efficiency internal gas system providing excellent FPS output, and reinforced valves and seals for optimal gas retention. We recommend using Valken Green Gas with this gas pistol magazine.