I shoot a Pedersoli 1859 Sharps Carbine in N-SSA competition.
I make up paper cartridges for mine, using the Eras Gone Richmond Sharps bullet. I have also used the Pedersoli bullet.
I tried nitrated paper, and indeed it does 100% of the time burn to ash, but I believe they are not as safe as non-nitrated cartridges. The reason is this: In the event that there is an ember in the breech, when you push in a nitrated cartridge if the paper touches the ember it WILL ignite the cartridge. Regular paper is more apt to smother it. Now both cases are probably rare because the cartridge is pushed in bullet-first, and this will probably push any debris/embers down into the bore ahead of the paper end. But still, there is no doubt in my mind that a nitrated cartridge is vastly more sensitive to a spark than a standard paper cartridge. Further, it's just not necessary. I go through 75+ rounds in a weekend and the bits of paper just get blasted to oblivion.
As for storage, I'm a big fan of MTM Case Gard's 20 Gauge Dryboxes. With 2 trays, they store 100 rounds of ammo. I like to take two pieces of cardboard and stick them in between the rows of holes on the bottom tray, to support the top tray, so the top tray does not rest on the ammo in the bottom tray.
http://www.mtmcase-gard.com/products/camping/dry-boxes-sd-100.php
The 20 gauge box works great for anything up to 58 caliber.
Black powder, especially graphite-coated black powder, is very difficult to set off with a static spark. There is an article on there on the web you can google about the fellow who made an electrically-fired muzzle loader. It was exceptionally difficult to get the black powder to go off. The reason is that BP is very electrically conductive. More so when coated in graphite. This means it offers little resistance to current, which means little heat is generated when the current flows through it. In fact, little current flows through it - most if it flowing
over the surface of the grains. This fellow even tried zapping the powder with a taser with no effect. In the end it took special circuitry to generate a spark substantially stronger and longer than your typical "doorknob" spark to make it go off.
Likewise black powder is not appreciably hygroscopic. Black powder was in use for centuries before the advent of air conditioning and when historic historic examples are discovered they are as combustible today as they ever were. As long as it doesn't get
wet, it will be perfectly serviceable stored in any kind of human habitat today, just like it was for centuries before air conditioning.
Steve