The Wikipedia has a write-up of needle guns with illustrations,
here. Interesting that the Dreyse version is effectively front (of powder column) ignition. Sending a firing pin through powder with enough energy to ignite the primer strikes me as asking for reliability issues, though.
These early cartridges mainly used mercury fulminate primers,
AFAIK. Fulminate can, indeed, deteriorate in a year or two under adverse climatic conditions. When you read the old timers referring to getting supplies of "fresh ammunition", better reliability and not simply a replenishing of depleted supplies is the reason.
My dad had some of those .50-70 cartridges in his bullet collection that I saw when I was young, along with a rifle that fired them. I recall the rifle had no kind of bluing or other protective finish on the barrel. Unfortunately, dad traded those off at some point. The rifle's barrel had been cut down and a curiously flimsy sheet metal sight added, so it wasn't in original condition, so I think he may have let it go a little too cheaply.