WB
I like, with a 180 gr., 20.0 gr. 4227. So
Saturday, July 21, 2018, 16:43

That gives me an idea where I'd start. Also I like using magnum pistol primers. It's not a max load. I'd figure 17-18 gr. of the same would be a safe beginning load. Cast bullets are lots more forgiving as to pressure spikes. However you need to take note of bullet pull during recoil. Some of the GNR rounds sacrifice neck length for the sake of performance with lighter bullets. I've not had any issues with the 180 gr. at all in my Blackhawk. Look at .357 Maximum loads for the revolver, back off a bit and try a few. That should prove safe.