I've got one of the Armi-Sports in .56-50 and load a 350-grain .515 bullet over about 35 grains of black in Starline .56-50 brass. I haven't recently chronographed these, but expect something in the 900s for speed. Another Spencer shooter mentioned he is using Magtech 32-gauge brass shotshells, appropriately shortened, and gaining 5-10 grains more powder space. Look forward to working up some heavier hunting loads with the the 32-gauge brass -- for possible deployment at cow elk under 100 yards this winter.
Tom, yes, I can get about 35 grains of Old Eynesford FFg into the Starline case under a 350-grain bullet, with moderate compression. Ordered two boxes of the Magtech 32-gauge stuff and can't wait to trim some to length and measure water capacity. The Starline holds 48.3 grains, if anyone cares.
The 32 Gauge Magtech brass arrived. I trimmed one and checked water capacity -- 57 grains vs. 48.3 in the Starline for a 15 percent gain. The Starline brass has a solid web at the case head, vs. very much like the old balloon-head case web in the Magtech shotgun brass. I expect the latter to be just fine with compressed BP for hunting loads. The meplat on that 350-grain .515 slug is .31. Wish it were larger, but that's still not bad.
Hold the phone on the case capacities. I full-length sized both the Magtech case and the once-fired Starline .56-50 case in an RCBS die. Resulting water capacities are 45.5 grains for the Starline and 53.7 for the Magtech -- a very useful 18 percent increase.