Re: Retumbo load?
Have a look at the Hodgdon site. There is lots of data now for the Retumbo powder.
From what I can see, performance is going to be very similar to Re22/25, H1000. It is only when you get to really heavy bullets, or overbore cartridges, that there "might" be an advantage.
This stuff is supposed to burn between H1000 and H870 so does provide an alternative to Re25. It should be temp. insensitive so would work well when developing loads for cases like the 300RUM and larger using heavier bullets, or very overbore cartridges.
For the 300Win mag and 190gr bullets, my guess is that the H4831SC, Re22, IMR 7828 and H1000 are going to give you equal or better performance out of a 26" barrel.