Pulled up this thread in a search. Very timely.
My own favorites for top end 357mag for the GP100 and Win94 center on H110 and jacketed 125 or 158gr.
But I finished out a bottle of H110 and thought I'd try some 2400. Also playing with oregon trail lasercast (158gr roundnose). Picked up a max of 14.0 gr of 2400 from one of the books.
My first tests at 11.6, 12.6, and 13.6 gr of 2400 seemed to be throwing a lot of unburned powder. After about 40 rounds, the cylinder of the GP100 would not latch and rotate properly. Cleaned around/under the ejector star, cylinder gap, etc with a dry toothbrush. Seemed to fix it.
Did not try any in the lever gun.
Was thinking maybe 2400 was not going to work with the hardcast. Would try it with jacketed bullets to see if the greater resistence in the barrel would build up pressure for a cleaner burn. Or just give up on the 2400 and go back to H110. I give 'em a good crimp, but I don't like to squeeze too hard on naked lead.
After reading this thread I may try 14.5gr with the hardcast.
Other than that, the velocity, standard deviation, and accuracy were great.
6 inch GP-100, 158gr L-RN oregon trail, 2400 powder
11.6gr..1177fps..sigma 17
12.6gr..1222gps..sigma 32
13.6gr..1282fps..sigma 25