Re: barrel length vs. powder usage
Retumbo has in recent years become my go to powder for my heavy bullet, large capacity LR hunting loads in 6.5x284 through 338 Lapua. While other powders work, I find that I get high velocity, low ES, low pressure, filled cases, and excellent temperature stability. I see no problem with burn rate effects using my 24-26" barrels. I believe Hodgdon did change it about the time they started labeling their powders as "extreme", claiming broad temperature stability. My loads at that time had to be reduced 2-3% by weight to develop the same velocity. They did something to change the formulation. It' seems to pretty consistent since then.
"Let us speak courteously, deal fairly, and keep ourselves armed and ready"-T. Roosevelt