I always suspected some of this was due to when factory loads were loaded with 7828 powder the same for the 243. I have never had any verification of this other than anecdotal experiences of others. Of course chamber differences and throat wear are additional causes. I have shot out one 7RM and probably didn't shoot more than a couple boxes of factory loads in it so almost no testing on my part with factory fodder. If it was case geometry then the 264 Win. should show the same variation, but the only inconsistency I have heard of with the 264 was due to two diameter bullets when compared to conventional single diameter bullets.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli