I've worked up a load for 2 of them, one of them being mine. Retumbo is fired out of both of the guns. I tried h-1000, and Ramshot Magnum also. I didn't mess with the Ramshot too much cause I found a load pretty quick with Retumbo, but I think my second choice would be Ramshot Magnum.
I got 91 grns in mine and that gives me 2800 fps with a 300 Berger.
im having a .338 Lapua being built by Phoenix Custom rifles as well. Allthough they are slightly different from all I have read H-1000 and Retumbo are the way to go. (Good thing I have 8-lbs of Retumbo on hand and access to more if needed because they use alot of fuel)