When I had my AR10 .308 I tried a four primer test and in that case, they made a difference. The cheapest primer wound up producing the best group. Go figure!
That was using a recommended recipe from the Alliant reloading page for AR Comp powder and the primer they recommended (CCI 200) wound up doing the best. One of the few times in my experience that a load recipe from a web page actually did what they said it would.
I miss that LAR-8 Predator HP, with its cryoed barrel, to this very day. Amazingly accurate.
Hoot