So... Some background.
- I only have 300MP powder. It would be great to get some faster stuff, but the reality is this is what I could find and what I have.
- I want to shoot some moderate loads through my Rossi 1892.
- And the bullets I have are Hunter's Supply 125gr cast. They claim they are good to 1600fps, but don't post a hardness rating.

What I would like to do is load up some low-end loads with the powder. But my concern is because it's a magnum powder, it might not load down very well (unreliable ignition, etc). Alliant claims 2,000 fps from a 10" barrel for a max load jacketed 125gr, which will be way too fast in a carbine. So it would need to be loaded a fair bit lower.

So, my thinking is to load it in a .38 Special case. I have plenty, and presumably the smaller volume will help the powder to burn properly. So to that end, I measured a 158gr jacketed that I have, and the 125gr I have. They are 0.13" different. The Hornady manual I have has a 0.14" difference in OAL for their 125gr .38 loads vs their 158gr .357 loads. So I am thinking the 158gr load data would be in the ballpark for a .38 case at .357 pressures. I.e. the volume of space in the case for both is very similar.

For a 158gr JHP .357, their range is 14.7-17.0 grains of 300MP. So I am thinking 14.7 grains might be pretty sweet in a .38 case with a 125 grain bullet.

Does the math work that way? And the bullet being lead, and also lighter, those things will both tend to lower the pressure all else equal? Other than concerns about the numbers on the headstamp, is this a dumb plan?

Thinking more about it, I have no intention of doing this. But I'd still be interested in feedback on the idea of it.

It seems that you have done some homework with this. Best way to start with a project like this, you will like it, is to shop for the most popular powder recommended for the bullet weight you plan on loading for the rifle. If you still want to use the powder you have start low and work your way up. You may find that the powder you want to use will not perform well in your rifle or it might do great. I have been reloading for 44 years and have quite selection of powders to choose from when working with any new load I want to try. Some work great some not so great. That's why we reloaded, to find out what works best. I do not claim to be a expert or master reloader. I touch the stuff everyday and I'm still learning. My best advice is to read and talk with others. If they recommend a load, for anything that you may be reloading for, approach it with caution. If you have good results with their info., talk to them again if you start working with another caliber. Best of Luck
Otis