I've tried a few and Alex is right, you may find differences and don't assume CCI will rule the roost. CCI 450 and BR4's work well in my 6BRA...Sellior and Bellot trumps them both in my rifle.
How does the thickness of the Sellior and Bellot cup hold up pressure wise in your 6BRA?
There is no best, but there is a best for your barrel.
My primer of choice has always been BR4. This y3ar I have got a bit more involved in 1000 gr8 gr8. I had ES of 11 to 22 out of my 3 competition rifles. Started tweaking with primers. Have ES now of 1 to 4. I tried all the primers. The answer in all 3 rifles was FED205. Just a report on what I found out with my rifles.I have shot CCI 400, CCI 450, WSR, Wolf, Fed 205, 205M,and 205AR in my Dashers and BRA's. The best accuracy has been with the Federals (205 and 205M) and the CCI 400's. The worst was with CCI 450's. James