I have some ammo loaded for another rifle and it is loaded with 40.5 grns of H4350. It does work good. But sometimes it’s hard to find. This is why I’ve tried to develop a good load with other powders. IMR 4451 also seems to work pretty well. My 12FV also has a Shilen barrel. The blank I used had a larger diameter spot that didn’t follow the taper of the contour on the barrel. That’s the reason for the 25.5 inches. I loaded some up with 41.7 grains of H4350 in reformed 308 Lapua brass but started getting ejector marks.