The ASC AR-15/M4/M16 Stainless Steel magazines are constructed with the highest levels of quality that guarantees durability and longevity! Their magazines are manufactured 100% start to finish in the USA using ".020" gauge 410 Stainless-Steel Alloy that is heat treated for superior strength, and lasting endurance. Stainless-Steel construction means they won't swell in heat or crack in the cold, making a more reliable alternative to polymer magazines. This is done all while maintaining that undeniably good look.410 Series Stainless Steel Body, Heat TreatedChrome Silicon SpringsEnhanced Anti-tilt Polymer Followers with PTFE300 Series SS Floor plate with Black Oxide Coating for Corrosion Resistance