The first test firings are now history. The loads with "real" 4320 charges in the middle of listed charges in the Lyman 46th edition held no surprises. Primers were flattened all the way across, bolt lift was effortless.
The loads with the mystery powder were a little surprising. They displayed lower recoil & noise, as expected, but also flattened primers all the way across and one round out of the five had slightly sticky bolt lift. There was no evidence of unburned powder in the bore which I was expecting from a slow powder/light load condition.
Temps this morning were in the 40s with high winds & driven rain, not suitable at all for setting my chronograph up. Velocity checks will have to wait for better weather.
I may try some even lighter loads, working downward until I find where the primers are not flattened. Probably won't mean anything but I'm curious.