AR's? Think about that one for a minute... then tell me how one welds the steel or stainless barrel to the aluminum lower receiver that it doesn't touch.
I'm just busting your chops. I know what you meant... you just happened to pick the absolute worst example possible.
Yea... soon after hitting "submit reply" I thought: "Bet he meant the lower receiver not the upper." And I guess it would be difficult to weld steel to 7075.