About my experiences shooting the rifle shown above:
after those first silly trials I did some reading, and learnt how to load a patched ball during a stay in the US.
About 15 years ago, I found a muzzleloading club in Switzerland. Some of these guys were really good, I remember at least 2 that won a European championship in a 50 m offhand competition. Preferred rifles were original Swiss target rifles, or similar replicas.
Starting with about 65 to 70 grains of Swiss No.2, I had cut and frayed patches. I gradually stepped back with powder to the volume of the mentioned powder chamber, ca. 40 grains of No.2 (3Fg), and settled on ca. 5 mm overpowder felt wad, a round ball from an RCBS .520 mold and a .015 patch. Patches were OK then, no blow-by visible. I never shoot a group over a bench at this club - they did not use any shooting benches, all shooting done offhand! But when I did my part, the shots sat were I had released them.
After a long pause spent with other things (job, family, hunting, modern rifles, clay shooting) I started shooting the rifle last year again, inspired by Zimmer's book.
First step in a very hasty test was to check velocity with the maximal load suggested by Zimmer:

.520 round ball, 200 grains with 53 grains Swiss No. 2 (3Fg): 445 and 453 m/s, mean 449 m/s or 1473 fps
.520 round ball, 200 grains with 53 grains Swiss No. 3 (2Fg): 412 and 381 m/s, mean 396 m/s or 1300 fps
.520 FN bullet from a Rapine mold, 307 grains with 53 grains Swiss No. 3 (2Fg): 377 and 361 m/s, mean 369 m/s or 1210 fps

So velocity was not quite as low as expected, and patches looked reasonable. I was not able to observe bullet placement, but all the different loads fired formed a diagonal string pattern (range was only 25 m, though).

So I think there is hope. Next steps will be to shoot formal 50 m groups, play with different patch thickness and powder loads.

This will never be a big game rifle, but good enough for our small roe deer (around 40 pounds, field-dressed).

Fuhrmann