I haven't tried the 300MP stuff yet. I was waiting for people like you to try it and report the results!
So far, I haven't heard any reports for my applications.
One problem trying a new powder is that it will take awhile for Alliant to give load guidelines for different applications. That is the case for the .45 Colt applications. Alliant doesn't realize that a lot of people shoot some really hot loads in their .45 Colts, so they don't get in a hurry to test those combinations.
If I was going to experiment with Ruger level loads in a .45 Colt using the 300MP, I would use the published data for .44 Mag as a starting point. In my past observations, putting the same amount of powder into a .45 Colt case using the same bullet weight as listed for .44 mag gave fairly similar results. Naturally you would reduce the starting weight a bit for safety but it would get you in the ballpark.
I had some great results using H-110 in super hot .45 Colt loads. When I first got my BFR in .454 Casull, I didn't have any .454 ammo and no .454 brass. But I did have a bunch of .45 Colt brass. I started out using Ruger only loads and they were fun. Eventually, I was shooting loads discussed by Linebaugh. Those were some high pressure loads no doubt. That is where H-110 really shines. It burnt very cleanly and consistently. According to Linebaugh, my loads were probably in the 45,000 psi range. Eventually, when I acquired .454 brass, I quit putting the hot loads into .45 Colt cases. It was kind of a pain to keep them separated from the normal pressure .45 Colt rounds for the regular revolvers I have.
It would be hard to go wrong using H-110 for hot loads in my opinion. Besides, for some reason, all the Alliant powders are higher priced than Hodgdon powders around here. In fact, around here you can buy W-296 is usually a dollar cheaper than the same stuff in a H-110 can. Go figure. And the W-296 can has a prettier label.
