Bill, a 30 BR at 300 has the same accuracy potential as a 6PPC at 300. The difference in the two would be which shooter has the best handle on the conditions.
I still think 300 is in “short range Benchrest“ territory, in that a good shooter can still get a precision read on his flags. That means the best bet is to use the most accurate rifle that is legal for the Match.
I would feel safe in saying that at any 300 yard Group Match, a 6 PPC will dominate. While higher BC combinations that are used in the BR variants might be better out further, they simply do not have the agging capability 6PPC or a 30BR.
But then, that takes us back o that whole condition reading thing.
A note about the Tack Driver. When I got home, I looked at all of my targets. In group, I started analyzing my score targets measuring each missed shot off of the center of the 10 ring and discovered that I actually shot better groups in the score portion than I did the group portion., if those five shots were overlayed. The majority of the “9’s” were just outside the 1” mothball. Two of them would have been sold .800 groups Which was pretty darned competitive.