Hodgdon Reloading on-line has LongShot data for the 500 S&W. They don't list Rainier's bullet but they have L-S load data for the Hornady 350gr XTP which is the probably the closest, most relatable manufacturer's reference you'll find.
Since you have the short barrel (3.5") revolver too you won't come close to their 10" bbl. velocity but I've never loaded over their starting load (21.0gr) for my max. with this bullet considering it's velocity limited plus you're deep seating it (1.965") a tad more than their COAL" of 1.985" listed.
I started at 17grs with this bullet & worked up in .5gr increments till I found a load I liked the most.
I began using L-S when you couldn't find Power Pistol, back during the powder shortage days. My only complaint with it is it leaves stubborn powder burn stains on the outside case mouth. It's less flashy than P-P. I've gone back to using P-P now that it's available.
Alliant/Speer has similar load data for P-P & a 350gr bullet.
Just a reminder, always DOUBLE CHECK the case fill before seating the bullet on reduced loads in large capacity cases like this. A double charge of these won't overfill the case (~75-90% fill) so you don't want to be careless & create a dangerous situation!
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For me, using Alliant powders as a reference, anything below Unique (Bullseye thru Green Dot) on their burn chart is fast. Unique to Blue Dot is moderate speed powder. 2400 & above is slow powder. That's the way I categorize them anyway.
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