Steel powder for a 450 bushmaster.

Reloader 26 is way too slow for a cartridge with an expansion rate as fast as the 450 Bushmaster has. You can expect the powder may actually extinguish during firing and leave a bullet stuck in the barrel.

As far as Steel goes, you could call Alliant to see what they say. Based on the relative burn rate being in the vicinity of Accurate N0. 9 and 4100, if I had to use it, I would develop the load using my Pressure Trace. I would run a few commercial loads with it and use their numbers as upper limits. I would start way low, like 20 grains for a 300-grain bullet, and work up slowly while watching for signs of pressure excursions or other irregularities. As long as there were none, I would head toward about 80% of the commercial load peak pressure and stop and then shoot it for a while just to be sure nothing strange came up before continuing to work the charge up further. The caution may seem excessive, but every once in a while, we get funny warnings, like the one from Alliant about not using Blue Dot in certain applications it used to be popular for. So you have to watch out for the unexpected oddity when exploring uncharted territory.