I've read of using coffee filter paper for patches on bullets, so I tried it with bullets and load of known accuracy; no. Back to onionskin. The description of results is 'patterns, not groups'.
Obtained some bullets cast from this mold, and tried them; not bad. I think these would work well with black powder, the lube grooves are deep and wide enough.
There's only so much wind in the face and glare I can take nowadays and still shoot well.