Just adding another data point. These are too short on my Pietta 1858 (2019 manufacture), but work just fine on my Pietta 1851 (2020 manufacture).
Honestly, I think my factory 1851 nipples are too long — I don’t think the hammer drops all the way before it hits them. I can give it a little push into position and feel the cylinder move forward ever so slightly. These newer nipples provide just enough clearance that the hammer doesn’t hit the nipple, but is close enough that it cleanly hits the top of the seated cap.
Edit: CCI #11 caps appeared to hold up better than the Remington #11 caps, but both fit readily and did not fall off.