Bullseye Smith
Member
As I have heard it, when it comes to ARs, the upper is the rifle. Not to say the lower receiver is unimportant, but the barrel and bolt (among other upper receiver parts) are the components that control much of the accuracy, reliability, and durability of your rifle.
The above is very true except for a match gun, if you hold on to the upper with one hand and the lower with another hand. Then flex the two like you were going to break it into. If you feel any slop in it you are talking inches on the target in a rapit fire match. The upper and lower must fit together tight.