I just published an article about all this. If you're not going to be shooting much beyond 500 yards, BC is such a minor factor it can be pretty much disregarded, given spitzer bullets. Many of the "low-BC," lighter bullets not favored by long-range hunters will drift as little in the wind as heavier, slower higher-BC favored for shooting beyond 500. The difference is even smaller, of course, at 200-400 yards.
But because of long-range hunting, many hunters have become absolutely fixated on high BC, even when it doesn't make any practical difference in wind drift at the ranges they shoot.
The OP states the guy building the 7mm RM is going to be shooting to a maximum of 450-500 yards. At those ranges 9and less) I'd rather have a bullet that expands and penetrates consistently when it hits stuff than a specialized long-range bullet that MIGHT result slightly less wind-drift, but may or may not perform correctly when it hits game.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
