I came across a load of #11 Magnum Caps for a Blackpowder Rifle. Is there any harm in using these instead of #10?
Thanks for your time.

As long as they fit over the nipple firmly and stay in place, it should be no problem. A #10 cap is typically smaller than a #11 cap. A magnum cap should mean that the cap is hotter than a regular cap and will likely facilitate the ignition of the black powder faster and with more heat. All of my black powder rifles have hot shot nipples on them that take a #11 cap. I replaced the nipples that came on all of them years ago with the hot shot nipple. That's my opinion.

Right, 11 is/was pretty much standard, but if they fit, use them. Even if they don't fit, you can use them at the range. But in hunting, they might fall off, a bad thing.
Harm? No. The term "magnum" means nothing in ML terms; some company might have put a bit more priming compound in them, but they vary among makers anyway. Back when we had multiple makers.

Thanks. Looking forward to my new rifle.

Pete, what is your new acquisition?