The Magpul Polymer Magazine (PMAG™) is a lightweight .223 (5.56 NATO), AR-15 / M-4 / M-16 magazine. The PMAG™ features a pop-off storage/dust cover that alleviates pressure from the magazine feed lips, allowing for storage of loaded magazines without risk of feed lip creep that often causes malfunction in standard aluminum magazines. PMAG™ utilizes a resilient stainless steel spring for corrosion resistance and has a window slot on the lower portion to visually verify the magazine is fully loaded. This is the HOTTEST magazine on the market right now! We use and recommend this item with confidence!