Capacity wise, it should come out about 76gr. water capacity. you can check this using H380 IIRC, which has the same specific gravity as water.

2,400fps might be a stretch for the very long 300gr., .338 bullet, but the others seem in-line. One thing's for certain, you can use top-end .338/06 loads for starting loads due to this round have greater capacity than a .338/06IMP. Every 9.3x62 I've measured has the Standard AI shoulder diameter of .454", not the .451 CIP measurement.

The 9.3x62 already has a considerably shorter neck than the .338/06, with the shoulder moved foreward. Is the neck of the 8.5x63 case even shorter than a 9.3x62?

The standard .338/06, .35 Whelen and .375/06 all have very long necks as they are simply necked up '06 cases. The larger caliber you go, the longer the neck becomes.

This is where Gord and I achieved up to 80gr. capacities (using WW '06 brass) in our .375/06IMP's - shorter neck and larger body, along with a 40 degree shoulder.