I ran across a powder, IMR 8208 XBR, that sounds about ideal for the 500AccRel. Does anyone have any experience with it in larger doses? (I'm thinking that I will want to use loads around 100 grains of 8208 XBR.

It is rated on a burn rate chart between H335 and H4895. Advertizing is always to be taken with a grain of saltpeter, but it turns one's head to hear a claim that from from 40 below zero to 165 F. the powder shoots the same speed. It does sound like one kind of a super pwder.
Here is the blurb:

"Introducing IMR® 8208 XBR super powder

January, 2010

For Immediate Release

Shawnee Mission Kansas- IMR® Legendary Powders introduces a new super powder, IMR 8208 XBR.IMR 8208 XBR where no target is too small is a remarkablyadvanced technology propellant perfected for the greatest match, varmint and sniper rounds known today---223 Remington/ 5.56mm, 308 Win./7.62 mm, 6mm PPC, 204 Ruger, 6mm BR, 22-250 Remington and similar calibers.

This short grain extruded rifle powder exhibits a previously unheard of consistency with virtually no change in velocity at temperatures ranging from -40 degrees F to 165+ degrees F. Uniformity from shot to shot translates into tack driving accuracy and this powder has passed the test:

Currently loaded in premium factory sniper type ammunition

Leading competitive bench rest shooter Mr. Lou Murdica won numerous matches with it in 2009, starting with Heavy Varmint Grand Aggregate at the Cactus Classic

August 2009 Mr. Jim Carmichel won the International Benchrest Shooters Association’s 40th Group National Championships, Heavy Varmint Grand Aggregate. Because temperature conditions change, bench rest shooters vary charge weights during a day’s competition. What is significant here is that Jim shot the exact same load of IMR 8208 XBR both days to win the event.
"