Re: Federal GM210M vs CCI BR-2 LR Primers
I played with different primers with my 300 RSAUM shooting 185 gr Berger VLDs. The velocity variation between WLR, BR2, GM210M, RP9.5M and WLRM was less than 30 fps. There was a difference in groups. I too was shooting from 200m, but I was using 3 shot groups. I got the tightest group with the RP9.5M primers.
With my 300 WM, I loaded up some 175 gr SMKs, and the BR2s shot tighter groups, but with the 190 SMKs, GM210M primers shot best groups. The 190 Berger VLDs shot best with RP9.5M primers.
Bottom line, you have to test until you are satisfied that you have done sufficient work on your "best" load. I would use the BR2s if they shot better. Your 1 10-shot group would be fine with me to switch, but I would continue to watch and see if it holds to make a pattern of supremecy. There's my opinion.