I'll be adding to this but in case people wanted to buy some to check it out on their own here's what I got so far.

Note that this was all done in a 20" Whelen with a short throat; about half the normal freebore and a slightly tighter throat dia than SAAMI.

As everyone has been saying this powder is definitely slower and it's definitely bulkier than RL15. Not positive it's a good fit for 250 bullets but I think it could turn out that way. For 280 A-frames or 310 Woodleighs it might be great.

Based on what I worked up with 310 Woodleighs it developed velocity and pressure about the same as Big Game and MR-2000. It maybe built pressure a tiny bit sooner than Big Game but they were very close.

Back to the 250's:

I used 250 hotcor at 3.35 COAL, in virgin norma/nosler brass (same/same as far as I can tell). I could only get 58 grains without crunching powder. That's after settling the powder with an electric toothbrush. It was way up in the neck.

58 grains of ole RL15 is a pretty warm load under a 250 Speer or Partition for this rifle: it gets a 250 partition motivating around 2540 fps, so I decided not to go any higher on round one for this RL15.5. Turns out I should have gone higher. Unfortunately, more than 58gr of this stuff is only going to fit in fired brass.

Anyway this stuff is accurate, I wasn't shooting for groups but most of the bullets from 55gr up to 58 grains went into basically the same group and it built velocity very linearly.

58 grains of RL15.5 under a 250 Speer hotcor topped out at 2450, about 90-100 fps slower than the same charge of RL-15.

55 grains of the 15.5 got 2335 and then it built pretty predictably up to 58grains at 2450.

60 grains will definitely fit in a fired case (3.35 coal) without compressing too much so that is next. I'm guessing 60 grains should break 2500 fps which for a temp stable powder is not a bad deal. But them cases are full.

It did fairly well with 310 Woodleighs but I'll put that in a separate thread.