Why not change the barrel on your .243 to a fast twist 7.5 or 8 and shoot any weight up to 115 gr. there's nothing
more accurate than the 6mm, and there's nothing in the lower U.S. or Africa plains game you can't take with the .243 the .243 may not sound as sexy as some of the others you listed but it will do the job of all of them Rio7
.243 with 100 gr worked just fine on moose, caribou, sheep, black bear for me.... Never really liked it tho. Still have that one.
I'd go .260 (did on a .244), 7/08 (what I was looking for when I found the .260 bbl), or .308.
