I've been pleased with VV550 in my 15" bbl, good velocity and accuracy. Varget and IMR 4064 are other ones that also seem to work well as with the above mentioned H-4895.
I have a Leu 2X mounted on mine and like it quite a bit, it isn't so much that you can't hold fairly steady without a rest although I use a neck sling. A 4X is ok if you have a good rest and better for target work. I think 3X would be about right. Havent tried a variable yet but....
Ryan