Those cut-down Springfields are nice guns, in my estimation. Sturdy, easy to maintain, and the drop and LOP is perfect for me.
One theory is that a heavy wad
under the shot column will blow a hole in your pattern, but I don’t think you’ll hurt anything by trying that load. I suggest looking up the Skychief load, as suggested by
@Sam squanch , but also check out
@Eterry ’s “You might be a redneck…” thread. I believe Mr. Terry used napkins from the local Sonic Burger for wads, and got good results. I haven't done any shotgunning recently, but generally used greasy leather wads, cut out of scrap veg-tan with a 5/8” arch punch, which is perfect for a 20 gauge. I was never very scientific about it, but just used two leather wads over the powder and one over the shot.
I would also recommend real black powder, if you can get it, or magnum caps if you want to shoot Pyrodex. Some of the cut-down Springfields, like yours and one I have, had the cone turned down for sporting caps, but yours likely still has musket-sized threads. You might try replacing that nipple with a musket nipple, provided you can get real musket caps and not just the “Reenactor” version.
Have fun with it! Those are nice guns.
Notchy Bob