Kinsman, you make eminent sense. I also am not a metal smith nor a metalurgist. But I am able to gather data, analyse it and detect trends. 6061 aluminum is used in billets which are then CNC machined into very nice receivers. Billets are more expensive to make because there is a lot more material to remove.
7075/7175 aluminum is about twice as strong as 6061 is and it is forged because it is not as machinable as 6061. Forging also strengthens the aluminum further. Forging is cheaper because less material is removed since the receiver is closer to its final shape than when starting from a billet.
I note that the companies who use 6061 in their 308 model receivers readily highlight the use of 7075/7175 aluminum in the AR-15 receivers. The ones that do not highlight the type of aluminum used in their 308 receivers just say "space age" or "aircraft grade" aluminum and to me that translates to 6061. Could it be that the AR-15 marketplace is more competitive than the "AR-10" one.
I completely agree the stresses placed on AR receivers are easily handled by either types of aluminum, but I also know that 7075/7175 is stronger (read more durable) than 6061.
Armalite alone produces the AR-10 and they are happy to tell you the lower receivers are made of forged 7175-T73 aluminum and the upper receivers are made of forged 7175-T74 aluminum.
