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.

Whaaat? I thought sure there would be dozens of Spencer shooters here!

Wish I had one.
Just load it full of black powder; you know how to do it. Whatever brass you have.

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.

For those unfamiliar with the .56-50 Spencer, it is essentially the stubby older brother of the .50-70 Government. that's a 350-grain .515 slug next to it.

fullsizeoutput_1014 by ComeWatson, on Flickr

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.

Well...
Let's see the friggin rifle!!

Heh.
fullsizeoutput_e43 by ComeWatson, on Flickr

Those are cool!
How are they for cleaning? Always wondered about that.

Not bad, in my experience.

Whose mold for the bullet? I'm looking to load for my 1865 Spencer.

Trying to recall. It's not the Lee 450. Will report if and when it comes to me, but I am getting to be an old fart ...