Yeah 25 years ago or more most of the non-Colt AR15's were milled billet 6061. There weren't any problems with as far as strength. DPMS uses 6061 for their 308 AR10's now. I preferred non-Colt lowers back then because they took the small front pin and surplus M16 uppers and the Colts had the large front holes and slotted screws. 6061 milled billet AR15 lowers are nothing new. They've been around a long time.