When the 6.5G was completely proprietary,back in 2004 or 5,I talked to Bill Alexander on the phone about this (HIS) chambering. He had initial problems with accuracy until he happened apon a certain powder.
I cannot remember the powder that he told me about but it was an "older" powder. Thinking there has been about 200 "new and better" powders pushed onto the market since then . . . including the military stuff,short cuts,etc,etc.
Thinking on it . . . I was talking to him about his Beowulf so discounted the Grendel powder recommendation.
Maybe BALC2,380,or 335 ??? ---- SAWMAN