I don't know how your rifling and shot cup are reacting to each, but soft or magnum shot will not hurt anything! I would think you could get almost a full ounce or at least 7/8 ounce load in the case without the shot cup. Use load data for 440 grains on 1 ounce and 385-400 for 7/8. Pick a velocity between 600 and 900 fps. (I really like 750!) Check you patterns at the distance your target will be, snakes at 12 feet seems logical to me.

Hint from trap shooters. Not only velocity influences pattern, but shot size also. The smaller the pellet, the bigger the pattern. That is why I use 7.5 (I didn't have any #6's at the time!) Also, larger pellets have more energy each, so the shot that do hit do a little more damage!

I had a single shot 4" derringer in 410 years ago. When I was patterning it, I was at 7 yards (for SD) and used 4'x4' card board to check with. (paper will do, but I had lots of appliance boxes). #9 factory loads were WAY bigger than 4', #4 was about 18 inches, the 3 buckshot loads were around 4". That gun would shoot 45 Colt too and would stack my cowboy action load as well as any of my 45 Colt revolvers!

Ivan