I had the same problem with my first batch of repurposed 5.56 brass converted to 300 blackout. About every 7th or 8th primer would pop out when fired. They would jam the bolt so it wouldn't cycle. I knew it was possibly due to a couple of issues. Since these are repurposed 5.56 brass they have a military primer crimp which you use a tool to remove. If you get a little carried away the primer will not be as secure... OR the load is too hot. In my case I believe I was using a very similar load to yours, so I backed off a grain or so and the problem disappeared. My current load gets me 1950 fps on my chronometer, which is what its supposed to be with the 150 gr. bullet weight, or thereabouts. The problem I had was there was ZERO info available for the bullets I was using. I bought them from Everglades Ammo. Nice bullets... I have 500 of the round nose and 500 of the boat tailed "pointy" ones. There is nothing to tell you that they are the equivalent of the XXX bullet or anything like that, so I had to pretty much pick a bullet from Speer/Nosler etc. that was similar in shape as a starting point. What I wound up concluding was the following after all of my testing.
The 150 gr. round nose bullets from Everglades Ammo are nearly identical to the Sierra Pro Hunter.
The 150 gr. pointed nose boat tail bullet from Everglades are nearly identical to the Sierra FMJBT GameKing bullet.
This was my starting point for my reloads. Using the load data from Sierra I loaded 20.2 gr. of H110/W296 powder (same powder...). This delivered 2380 fps on the chrono and would blow out the primers on a regular basis. The 2380 was very close to the "recommended" Sierra load of 20.5 gr of H110 that delivers 2450 fps. So I knew I was in the ballpark. They refer to this as their "Hunting Load" vs the "Accuracy Load" which is much lighter in powder weight. So... I compromised with 18 gr of H110/W296 and it delivers 1950 fps. with good accuracy. No more popped primers! I use this load for both bullet types. For the C O A L I'm using 2.057 for the pointed bullets. On the round nose, they are a lot shorter obviously at 1.846. The Sierra info states 2.055 and 1.845 respectively. Its probably not the optimum for a hunting load but if I were to go with a hunting load, I'd get new brass and use the 20.5 gr. powder load. After all of this I concluded that I was over reaming the primer pockets when removing the military crimp, which in the back of my mind I was concerned about because it was so hard to be consistent with it. I changed to a primer pocket reamer, as opposed to a cone shaped cutter, that doesn't allow you to over do it quite so easily, if at all. So, you now know what I know.
