300BO subs work best with AA1680, Lil’ Gun, IMR4227, and other powders designed to work in magnum pistol and intermediate rifle cases along with smaller shotgun shells.
Don’t think any of those would scale well for this.
I standby the idea of filling the case to just below the base of the bullet with Trailboss. TB doesn’t like to be compressed, but it gives excellent case fill and low pressure.