FWIW, I have a 243 that is very cantankerous. I knew it had good potential because it put Blue Box Federals into very small groups, but of course I wanted to use my handloads. Tried all the usual combinations without success. On a whim, tried some IMR4320 that was on the shelf, with some old 100gr Nosler Solid Base bullets that I'm hoarding. Bingo, instant small groups!
PS 4320 is small grained and meters very well...
