Re: Which primer to use
I don't worry about primers until I have a reasonably accurate load figured out, then I'll change just that and look for a difference. In my AR I haven't seen any of significance, in my .308 bolt gun 1/4-1/2MOA differences with powder/primer combos. The only reason not to use a given primer in an AR is if it causes doubling. I have not had this problem in mine in 4500rnds with any brand, but I suspect its on the ragged edge, the firing pin dents the cups on chambering.
The Bergers were free, shoot the hell out of them, maybe some combo will shoot really well, share it if you find one.
Depending on your expected distances something other than match bullets may perform better. My 1/9 16" shoots 60gr VMaxes better that anything I've tried short range, 3/4" groups at 200yds.
What's your bad experience with Sierra? Accuracy is a product of shooter, weapon, and loading, every load won't perform in every weapon. Milspec accuracy standards aren't very stringent and in the M4 a rather large degradation is allowed before its unservicable. SMKs were never intended for good terminal ballistics, though they often work well nearly all match bullets are considered unreliable, they simply weren't designed to consistently penetrate and/or expand.