Stormin.40:
I think you are taking a prudent approach to your load development. Regarding H110, I'd like to add some info about my experience with this powder and .357 Mag loads.
When I could not find 2400 any more I decided to switch to an alternate powder. H110 [
W296 is a near clone] was available and highly touted so I decided to give it a try. It does not have as broad a load range as 2400 so you are limited as to how far you can
download with it - below a certain point [
load density / pressure range] it will not burn efficiently - and Hodgdon warns against this. You will need to use magnum primers to get good ignition and complete combustion. A firm roll crimp helps to build initial pressure and efficient burn. Velocities plateau sharply at the upper end as a function of powder charge. Performance consistency is quite dependent upon load density [
efficient combustion] so the best consistency occurs at the upper end of the load curve.
My best loads using H110 for 140gr and 158gr XTP bullets for 5 shot strings from my S&W 686+ 4" are as follows:
140gr XTP:
18.5gr H110, WSPM primer, S&B cases trimmed to 1.278", COL of 1.580", firm roll crimp, MV of 1326fps, ES of 29fps, SD of 13.2fps
158gr XTP:
16.5gr H110, WSPM primer, S&B cases trimmed to 1.278", COL of 1.580", firm roll crimp, MV of 1192fps, ES of 29fps, SD of 12.3fps