Thanks for the posts, gentlemen. Great information!
H4350 seems to be a universal performer. I use it, and have used it, in a lot of cartridges with great satisfaction. When I do get this rifle, I want to give some slower stuff a try as well, namely Retumbo. I have had excellent performance with that one in several cartridges, including 264 Win mag, 7mm Rem mag, 30-378 Wby mag, and 300 RUM. I understand the 243 Win and 6mm Rem cases are not as large, but they are
quite large relative to the 6mm bore size, and the heavier 115gr slugs will likely benefit from the sustained pressure of the slower powder. If H1000 works well, as Formidilosis has said, then Retumbo should work at least as well, if not better. Another impetus for using Retumbo is that I have a supplyy of the stuff on the shelf leftover from my magnum