Your pressure peak is within a couple of inches of the chamber. I've read a lengthy article by Mic McPherson and a shorter one by Ross Seyfried explaining why the best powder is the same one, regardless of barrel length. Made sense to me. You might get a bigger fireball with that short tube if you use 7828 instead of 3031, but you'll get the fps too.
I'm not sure if you take it to extremes, like a .357 Magnum in a 2" stubby vs. a 20" Marlin.
Yes, the load is going to be 180gr FailSafes from a 300 WSM out of a 20" tube. The only variable is going to be the powder. Since I am handloading it, I thought it might be efficient to try a powder more appropriate to a short barrel round like a 30-30 or .308.
I would not move to a faster powder, myself. In addition to the writers who recommend against it, I did some extensive testing with a 20" barrelled .243, which actually has greater case capacity when compared to bore area than the WSM. I got best results with some of the slowest powders, including H4350, R22, and IMR 7828. Big fireball at dusk, but also the highest and most consistent velocities. IMR 4064, 3031, and H4895 didn't even come close. This experience was with 100 and 105gr bullets, not varmint weights.
I think you'd be better off with something like H4350, myself.
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