Back to the OP
I use R26 in my 243AI and FWIW I have found it almost interchangeable with H4831SC with near identical velocity with the same charge and bullet weight. Bullets are 105 and 107 grain match bullets.
I know that probably won’t translate directly into other cartridges or even other bullet weights with the same cartridge, but in that narrow range of application R26 and H4831SC are very similar.
48.5g of either powder makes about 2980fps in the 21” barrel with the Hornady 105g BTHP.
