[Top 10] Best AR-15 Upper Receivers in 2021 - Complete
Uppers are almost always made from aircraft-grade aluminum (dating back to Stoner's original work with the Fairchild Aviation-owned ArmaLite Rifle Company) and typically come in two grades 6061-T651 and 7075-T651 with the latter being preferred as it is near twice as strong when it comes to tensile and yield strength for very little ...
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