EDIT to add pics and adjust group size.
I did shoot the Spencer this week. Worked great! I was prepared for some finicky feeding, but didn't experience it. The Buffalo Arms ammo worked well through the action and shot OK. I shot 39 rounds...saved one box plus one round so I can measure them for length. bullet diameter, etc. Since they fed so well, I will use their length for a starting point when I load my own. I also want to take the one round apart to measure the bullet and powder charge.
The BA ammo is loaded with BP. The real thing, I think, based on smell and cloud of smoke. At 25 yards from a bag, I shot one decent group of 5 into just under 2.5" (with center hold, it was about 5 inches high). See the RED Group 1 in the pic below.
Then I moved to a 50 yard target and put 12 rounds into a pattern about 12 inches wide by 6 inches high. Much of this can be attributed to the coarse sights and difficulty in holding a fine sight picture. I think.
Then I went back to shooting at 25 yards and got results that were...odd. At 25 yards, any carbine should be able to hold a fair group, right? Sight alignment didn't seem like a problem. I changed to a 6 o'clock hold and squeezed the first two into about an inch nicely centered and just above point of aim. Then then next 3 using the same hold and careful firing ended up at the edge of the target 6 inches away, but all within an inch! WTF? I continued firing with the same care and sight picture and ended up quite a few more in a good cluster at the left edge of the target, but also other scattered between there and the original 2 shots. Again...WTF? Looking at the target after all was said and done, I COULD draw circles around 4 groups of 5 shots and pretend it was on purpose. But it wasn't. 20 rounds at 25 yards from a front bag rest and the group size is like 7 x 7. See the GREEN Group 2 in the 25 yard pic above.
The way the impact was moving around, I thought maybe the sights were loose and jumping from one place to another, but that doesn't seem to be the case. The hammer WAS a little loose, but would that account for a big change in horizontal impact? Other thoughts are that the ammo is inconsistent; the bullets too large or too small; the fouling build up was to blame; heat build up to blame; barrel pressure from the foreend.
I'll come back and add some pics of the targets.
Rob