I haven't loaded Rainier bullets, but I have run 5-700 rounds of Berry's 180 plated bullets with 800-X. They are very similar types of bullets.
The starting charge you list there might be a bit high. Hodgdon's max for a jacketed bullet is 8.7. The Rainier is a plated bullet. Rainier recommends using data for lead bullets which will typically be less than jacketed recipes. If lead data is not available, you can back off max loads for jacketed bullets by 10% and that would be the max for a plated bullet. Then reduce from there to begin load workups.
Berry's recommends limiting loads to about mid-range charge weights called for in recipes for jacketed bullets. So weighing recommendations from both manufacturers, 8.3 grains is a bit warm for a starting charge for a 180 gr plated bullet. I tend to think that Berry's load recommendations makes good sense as I have experienced with a few different powders.
That said, you'd likely be fine at 8.3 grains. I would not start there. Reloaders should make it a practice to work up to the goal charge weight. I would be inclined to make a strings at 7.6, 7.9 and 8.1 grains and check for any issues before shooting the 8.3 grain loads. Velocities for the 8.3 grains should be about 1050 fps. 7.6 was 1000 fps, and 7.9 about 1020 in my G20.
8.5 grains is the maximum I will shoot in my G20 with a jacketed bullet because I get symptoms of excessive pressures beginning at about 9.0 grains.
Plated bullets do not like to be driven hard. So that is something to consider too. 1050 is not being driven hard by 10mm standards, but I have found that to be getting close to the upper limit for these thinly-plated bullets.
Notes from my log for 180 gr plated bullets and 800-X state that groups were good up to 8.0 grains and less and gradually get worse beyond 8.0 grains. It was very dirty as well.
A few loading tips for plated bullets:
The plating is very thin and shears easily. That can cause bad accuracy and even cause the bullets to tumble.
When expanding the case mouth, give a generous bell.
Seat and crimp in separate steps.
Crimp only enough to iron out the bell. You do not want the case mouth to engrave the plating at all.
FWIW, my best results for 180 grain plated bullets have been with Blue Dot powder.
Good luck. I hope it works out well for you.
Welcome to loading for this awesome cartridge.
Last edited by Taterhead; 02-26-2012 at 00:29..