Rather than speculate about what powder charges to use, go to your reloading manual and look up .357 Magnum loads for 120/125 gr. cast bullets and start there, since you're loading plated bullets. Just a quick look in the Lyman 48th, which is the one I keep by my computer, gives a bunch of loads in the 900 fps range as starting loads with several different powders. You can also load in .357 Magnum brass below the start load for the bullet you're using, but keep it in the upper range of .38 Special loads. That way, you don't have to worry about sticking a bullet in the barrel.
Don't be afraid to experiment with different loads in this caliber. The load I use the most isn't even specifically listed in any manuals. I just experimented with different loads until I got what I wanted and stopped right there for the bullets I cast the most.
Trying to extrapolate can get confusing sometimes, and the formulas don't always work out. I'll take one example to show you. In the Lyman 48th, it shows a load for a 120 gr. cast bullet with a maximum of 4.6 gr. of Bullseye for the .38 Spl. which gives 1001 fps and 16,400 C.U.P. For a 120 gr. cast bullet in .357 Mag., it lists a starting load of 6.5 gr. of Bullseye which gives 994 fps and 18,800 C.U.P.
The .357 Mag. case is 1/10" longer than the .38 Spl., so it takes more powder to make up for the increased volume to get a similar velocity for the same weight bullet. But notice in the above example that to get nearly the same velocity, the pressure was 2,400 C.U.P higher in the .357 Mag. for nearly the same velocity, and the lower pressure of the .38 Spl. actually averaged 7 fps higher. It could have been due to different test vehicles, or other factors, but you get the idea.
Hope this helps.
Fred
