Evolution Gun Works Main Hammer Spring ISMI 17 lb. All ISMI Springs are made from silicone chrome instead of 150 year old music wire. Every ISMI spring is heat treated shot peened & stress relieved to enhance durability consistency and reliability. ISMI is a contractor of the U.S. government and produces springs used in machine guns with incredibly high cycling rates. Also the creator of ISMI springs obtained his knowledge from extensive research in the development of springs used in Indy car racing. Specifications for Evolution Gun Works Main Hammer Spring ISMI: Manufacturer: Evolution Gun Works Color: Black Package Contents: Evolution Gun Works Main Hammer Spring ISMI What firearm models does the manufacturer say Evolution Gun Works Main Hammer Spring ISMI is compatible with? The manufacturer suggests that Evolution Gun Works Main Hammer Spring ISMI is compatible with Colt Officer's ACPEvolution Gun Works 10640: Main Hammer Spring ISMI 17 lb