So I finally got my RPR out to test it out and I found the same thing going on with the primers that Cobra is having. I shot 30 rounds of factory ammo and 48 rounds of hand loads. My hand loads were all at or below published max loads using new Lapua brass, CCI BR primers, Varget, or Benchmark Powder, and 75 grain Hornady Match or 80 grain ELDM bullets. The factory ammo I shot was all 55 grain ammo that varied from Wolf Gold, Fiocchi, Federal AE 223, CBC M193, and Gecco. With the factory ammo, all the rounds that chrono'd over 3200 ft/sec had the serious cratering problem that Cobra is seeing. Wolf Gold and CBC were the worst of the group of factory rounds. This is the only rifle I have that I've noticed this with. And honestly it's a cause of concern for me.
Now for my hand loads, I never really got up past 2680 ft/sec with any of my loads, so the cratering wasn't as pronounced but I did notice that it started. Overall, I was getting really good accuracy with Varget. Benchmark and the 80 grain ELD didn't work, and Benchmark with the 75 grain HM was better, but not ideal. All shots were at 100 yards. I averaged 0.587 inch groups over 12 rounds of 4 different charge weights using 75 grain Hornady Match Bullets and I averaged 0.607" groups over 12 rounds of 4 different charge weights using the 80 grain ELDM. My best group measured 0.157". Note that these were all 3 shot groups, but I don't have the time to reload or shoot 5 shot groups for initial load development so I go with 3 to get an idea of what velocities look like at given charge weights. Considering the above averages were over 12 rounds, I feel like that is a good indicator that the rifle is a shooter though.
Using the 80 grain ELDM, a CCI BR4, Lapua Brass, and 24.0 grains of Varget, I was getting an average velocity of 2622 ft/sec. I'm really looking to push up the charges as I'm not getting the velocity that I'm looking for, but I'm reluctant to do so given the primer cratering issue. I think it kind of sucks that the RPR is listed as chambered for 5.56, but has significant primer cratering using 5.56 pressure factory ammo. I don't see this with any of my AR's or Remington 700 shooting the same ammo. I will post pics of the factory brass that I shot tomorrow.