Yesterday I tried the homemade caps I made last week in my Pietta 1860 Sheriff which has stainless steel Slixshot nipples installed. My results were as follows:
6 caps to clear the nipples: All popped. I noticed they make less noise than CCI, Remington, or RWS caps.
6 shots loaded from paper cartridges with 25 grains of Scheutzen 3Fg BP: The first 3 went off OK but the next 3 popped but failed to set off the BP. I recapped those cylinder and none set off the powder. I recapped once more using RWS 1075+ caps and all 3 chambers went off.
They may work better with paper cartridges if I increase the amount of priming compound in each cap.
6 shots with 25 grains of Scheutzen loaded from a flask: All 6 went off properly.
6 shots with 25 grains of Triple 7 3Fg BP substitute, which has a higher ignition temp: All 6 went off properly.
The caps on shots with 25 grains of BP held together but flowed a little back into the safety notch in the hammer nose.
The caps on the Triple 7 loads all perforated but the skirts held together.
None of the caps fragmented and I had no jams. Some came off easily while others had to be pried off either with my fingers or pocketknife.
Overall, I consider the home made caps using the Prime-All compound a success for use in percussion revolvers and I'm sure they'd work fine in an underhammer. I need to test them in a sidelock rifle or pistol.