I'll start with the obligatory reloading statement: These worked with my rifle, make sure you do your own work-up. That being said, I did follow TexasSheepDogs advice, starting low and not exceeding 42 grains LilGun.
I recently bought some Rocky Mountain Reloading MHI bullets as cheap plinkers in the 450. I've had tons of trouble with extraction using the 300 grain bullets, so I figured I'd try something new. I was only testing for pressure signs, velocity, and ability to cycle, I promise I'll get some accuracy testing done at 100 yards in the future when I get more time. All the rounds were shot standing off hand, 10' from a the chrono at a distance of 25 yards.
Rifle: 18" mid gas, no brake
OAL: 2.07
Taper Crimp: I started at .476" and reduced to .475" after the first string up to 39 grains. I may decrease even more to .474 due to multiple bullets "setting back" when not cycled properly
Powder: LilGun
36 2066
36 2104
37 2192
37 2047
38 2156
38 reference picture
39 2231
39 2302
39 2174 .475" crimp here on out
39 2180
39.5 2305
39.5 2345
40 2378
40 2343
40.5 2325
40.5 2531 WHAT?! had to forcefully extract with charging handle, but no other pressure signs
40.5 2364
41 2376
41 2490
41 2431
41 2337 (shortened OAL due to bad cycle)
Trends: I noticed that every round extracted with one exception. Almost all the brass dropped directly to my feet when I had a good cycle. From 38-40 grains, if I loaded 2 in the magazine, the first would extract and jam with the second (reference picture). Recoil was pretty mild. I never noticed a flattened primer, ejecter or extractor mark. With the very limited amount of rounds per powder weight, I think I will begin at 40 grains for the 100 yard testing and tighten down the taper crimp to .474. Either I started shooting better or the groups started tightening up as the powder load went up, no way to tell, but I'll put pictures up anyway.
Lastly, if you haven't checked out RMR, DO IT! I've ordered tons of bullets through them, they are an awesome small business that treats their customers right. I've never got a bad product from them. Shameless plug over.