I have built several ar15 pattern rifles, starting on my first ar308.
So far I have bought a aero m5 lower and a prebuilt m5 upper with a 20 inch heavy barrel.
I bought the upper from delta tactical, I have bought quite a bit of stuff over the last few years there and the prices seem to be good.
I paid $234 for the M5 upper with 19 inch ultra slim hand guard, 20 inch 416r heavy barrel. I figured go big or go home
I read about all the troubles people were having with magazine and bolt catches, so I ordered the mag release for a sig 716 and it is about 1/4 inch longer than a standard catch and it is ambi.
I haven't decided which BCG to get yet
thanks, Mitch