I was able to do a modified 'Audette ladder test' the other day with Retumbo in a 7mm Remington Ultra Mag. The powder itself is somewhat finer than many boomer powders, so it should meter fairly well. It did take 1 1/2 months to arrive on backorder, but that's life.
Wind: 10-20 from 5:00 to shooter
Humidity: 98% with light rain
Action: Remington LA
Barrel: Kreiger 30" 1:9" twist heavy Palma taper, no muzzle brake
Stock: McMillain Prone w/3 way buttplate
Trigger: Jewell set @ 1 lb.
Scope: Nightforce 12-42x56mm,set on 40X
Bases: Champion's Choice 30MOA
Rings: Badger Ordnance 30mm
Brass: Remington 300RUM necked down to 7mm & fireformed, prepped but unweighed
Bullets: Cauterucio 176gr VLDs seated .005" into the lands
86.5 Starting load - fouler
90.5 Stiff recoil, primer cratered, minor stamping
91.0 Stiff recoil, primer flattened & badly cratered, heavy stamping & bolt lift - TIME TO QUIT & BACK DOWN!!!
All shooting was done at 200 yards. The total size of the group (excluding the fouler was 2.049." The three rounds at 87.0, 87.5, & 88.0 grouped at .570." The three from 89.0, 89.5, & 90.0 grouped at .984."
Recoil & muzzle blast was noticably heavier than max loads of 5010 that I'd used last year with good results. Obviously, the next phase of testing will involve groups at longer distances with powder charges of 87.5gr & 89.5gr, in addition to a chronograph to check velocity.
All in all, inconclusive, but promising... This rifle is primarlty used in conventional LR competition, but will occasionally be used in LR coyote & deer plinking. The 5010 pulldown standard goes out at 3,360 & ES & SDs in the low teens. Average 500 yards groups for it with 5010 were @ 3.25" last year. At $4/lb, the 5010 is tough to beat for that performance, as opposed to $18/lb for Retumbo.
I tried 50BMG last year, & unfortunately it did poorly. Some of my buds have had good luck with it in different species of RUMs, but this 7RUM hated it. There's not much 7mmRUM data out there for ANY powders.
Others I've tried are: RE22, RE25, WC860, MRP-2, IMR7828, & H1000. The 7828 wasn't bad, but only the 5010 did well enough to go the distance. I tried Caut 156s & 176s, JLK 180s, & Sierra 130s for kicks.
I'm surprised that you find the retumbo to be a powder of choice in that gun. I'd say H4831, H414, N160, W760, IMR-4831 would be better choices. How in the devil are you getting that much powder in the case? I would have guessed that 65-70gr of say, 4831 would be full or compressed, is retumbo that much more dense?
Shoot the gun at night and see how much flame exits the muzzle. If it's a foot or more, I'd choose a faster powder and less of it.
If you have something that you disassemble and reassemble enough times, sooner or later, you'll have two!
200 yards is the absolute low end to try and do a ladder test. You can really tell the differences when you go to around 300-400. If you do it late in evening or early morning (no wind condition) it really makes the stringing and grouping definitive.
If you have the distance try it and see the difference.