Anyone have loading info for .450 NE with RL 15. Using 480 and 500 grain bullets.

Graeme Wright says 84 grains, I tried 80 to 84, BUT, it won't regulate in any of my 3 450s; recoil is not enough to make the bullets come together; they all crossed badly; I have to use 4831, much more of it (96 grains) to get regulation. Might want to use a filler too but be careful.

I was going to try RL 15 to reduce recoil but to get the velocity of the regulation speed needed at least for my Heym in Graeme's book page 209 the pressure is much higher than with H4350 or 4831. Interesting that yours are crossing which I'm sure mine will as well. Oh well that's one of the reason to reload, you learn things.

Taking on a Buff hunt in TZ in August so I need it to be right.

I really appreciate your input and taking time to answer.

Which is why speed alone is not the sole determining factor in double rifle regulation. It is how the bullet gets to that speed that matters more. You need the recoil to make the required muzzle flip and jump (those are tank gunnery terms) to make it regulate, again, depending on how the maker regulated it in the first place. For example, my Searcy was regulated with 96/4831/480. Nothing else will work and RL15 crosses badly even at the same velocity. It does kick a lot less.....which is a bad thing in this case.
My goal was always to get a cape buffalo. Pretty sure it won't happen...

Well I'm hoping you get that buffalo.

Mine shoots very well with 95 to 97 grains of IMR 4831 with 480 Woodleighs but going to 500 A-Frames as my PH thinks the Woodleighs are soft.

M-48 Fort Knox 1971 training.

If yours is regulated with 4831 then you can try it with RL15 but as I said, I have tried that on 3 rifles and it didn't work.
I started at Ft Knox in 1973 but we had M60A1s then. I chose the wrong career path to get a cape buffalo. No time when I was in the Army, (28 years), and now that I have time, well, it won't happen.