Usually RE50 is used for the heavier 800 grain bullet weights at 240 grains and above with a RWS primer.
For the lightweight bullets you are use H5010 H50BMG or VV24N41 would work better starting at 215 grains.
You don't build enough pressure and it fouls the barrel sooner.
US869 when it first came out was supposed to be the slowest powder on the market intended for 800+ grain bullets.
I find it shoots better with the lighter bullets than the heavier bullets which is an exception to the rule.
Welcome! We have one helluva community here, it's definitely a unique place.
I load these in mass numbers. I'm working on a lot of 10,000 cases (and have been for a while, shit keeps getting in the way!). I use a Dillon BFR. Turns out there are other, smaller mfg.'s and someone even makes a 650XL-like press (not Dillon). The Dillon press is solid though and that trimmer is the only way to go IME.
For Ma Deuce belted ammo, I'm using 240 or 242 grains US869 (I'd have to look) under API's. I haven't started loading for accuracy in the HTI yet, got a divorce, surgeries and other shit keeps getting in the way. Just mispec copies for the most part, but I've got almost 1000 belted up.
It's hard to find much data on loading for these though. And pulled BMG bullets come in all weights (and usually the data you need is for the OTHER one!). Some one said they quit making H50, I've got a jug of it but I've got closer to 80+lbs. of US869 --I'm hoping it lights the tracers up. I've heard they can be sketchy and the military data all calls for an obsolete powder made by IMR (5010?). I was in contact with a guy that can get mass quantities of military powder but all he could ever get was 20mm powder in 20lb kegs. Lots can vary a lot too and I've heard some horror stories so I just stuck with the US869. It ain't cheap but at least I know what I'm getting.
My API's are 647 but the APIT's are 619. What a pain in the ass. If it wasn't such a chore to take it out and find a suitable range, I'd probably have it all figured out by now. For the M2 it's not so important, but it's a different story with the rifle.
Trainman here. shot the 50cal Barret 20" barrel today w/239.3 gns. of reloader 50. Read 2596 fps on chrony , primar was quite flat . The case resized very easily. Will cut back to 230 gns. By the way it was an API 648 gn bullet. This info may help someone else. Later