How much H335 behind 123 gr SST?

[Harpoon1]

Do you mean you use the high side of the Hornady range?

If it were me, I’d use the Hodgdon data for the SST’s. I believe the difference in charge weights is, the Hornady manual includes the GMX/CX all copper monolith bullets along with traditional cup and core bullets in the data. That means they have dial the charge weights back to accommodate the monoliths, which are harder to engrave in the rifling. To my knowledge there is no official “bolt gun” load data for the Grendel. The 6mm ARC is another matter.