.338 LM / US869 / 285 AMAX

Mine does pretty well with 300 gr. Scenars. Between 105-106 grns of US869 works well for me but it's not temperature stable, was getting ridiculous velocities and some mild pressure signs this summer at 102-104 degrees. You can work up some great loads with it. It's a bit of a hidden gem being that every other magnum powder is unavailable. I would grab it, because it does work fine with 338LM. Also might try Ramshot LRT, haven't had too much time to develop loads with it but seems more readily available.
 
I almost forgot, you can try IMR 7828 as well. It burns super clean. I have the SSC (super short cut) variant and leaves a lot of space in the case. I would go with the standard 7828 if you can find it.
 
Buy the RL33. You may run out of case capacity before you max out on pressure, but it's a good powder. If that doesn't suit you, I know there's plenty of N560 around. A little spendy but that's a great powder, runs about the same as RL26 for all of my loads.