I wanted to get any thoughts I could about polymer frames on the bigger AR's. My friend thinks the 6.8 will tear up the lowers. The gun store by me says the only issue there should be is the way it will look. The only reason I ask, is I can get a RRA upper with a plumb crazy lower for $660, and I'm a budget shopper. It will only be an extra $40 for an aluminum frame, but like I said, I'm a budget shopper. Any thoughts?