On my '62, with Treso nipples, the magnums will not fire, even with repeated blows, and being pushed on the nipple with a push stick. After repeated blows, I can see "mushrooming" of the cap, as the hammer does not seem to have enough force to fully seat the cap. I suspect they are made of a thicker gauge of metal. Regular CCI #11's, Remington caps, etc. all fire reliably on both the Treso's and the Uberti nipples.
The magnums also work fine on my Plains Pistol, so I suspect perhaps maybe possibly they are intended for rifles/sidelocks? I think they work on my Remington Navy and 1860, but those are both loaded right now so I can't snap some to make sure. Actually, I just recalled, (!!!) that they seat so far out, or high on the 1860 that they get closer to the recoil shield than I am comfortable with....so, haven't tried them on the 1860.