This Enfield 4-Mk I (SMLE) rifle 10-rd magazine's body and follower are constructed of heat treated blued-steel, with a chrome-silicon wire spring to ensure reliable feeding and function.ProMag magazines are designed utilizing traditional stamping and welding of heat-treated high carbon steel. Every ProMag magazine is backed by a lifetime guarantee against manufacturer's defects. Features: Magazine body constructed of heat treated carbon steel with black oxide finish Stamped steel magazine follower Magazine spring formed from heat treated chrome silicone wire Made in USA