I will throw in an interesting experience I had loading straight wall 357mag for both a 5" revolver and a 16" rifle.
A while back, I picked up a pound of 300-MP to try. I noticed it had significantly higher velocities in the published load data than the other magnum powders. However, when I worked up loads to test in my 5" GP100, I did not see higher velocities than 2400 or H110. And, working up in charge level, the velocity peaked at 1200fps and did not increase AT ALL starting at ~ 2.0gn below max charge. That last 2.0gn range all were ~ 1200 fps. (158gn XTP/mag bullets). I was confused by that.
A few months later, I picked up a 16" Rossi rifle, and thought I would give 300-MP another try. This time, the velocity increased all the way through the charge range, right up to the max. The most accurate load was about 0.6gn below max charge, which gave me 1800fps. This is > 200fps faster than 2400.
I went back and tried to understand the published load data, and Alliant had published numbers shot from a 10" barrel. Hornady I believe had used an 8" barrel. Evidently, 300-MP works great in long barrels, but has no advantage in short barrels.