Lol… yeah I don't know either. I was told that (rule) years ago and have just followed it and it has worked out for me, but I do know the the LRP work fine in a pinch…Never saw a 60gr rule, but anyway I don't use MRP. I have tried them in year gone by. I have found over the years the Fed 210 work quite well. I do have a stock of different primes across the board at the same time. In working up a load I try different to see if that any gain in a different primer. I loading in the 75gr areas. with my belted mags. I do feel that over 80+ grs the MRP probable should come into play.
There is a lot of back and forth on primers, anyway.