Falling,
I've made several, empty cartridges are things I've usually got plenty of!
I've found the best heat source is a tealite or candle, holding the empty vertical above the flame so all the heat goes into the metal. Some brands of cartride just dont seem to want to come apart as easily as others. - Oh, make sure its a fired case, not just one someone has cut the end off, if you cook off a primer it'll ruin your day!
I do like this matchbox, it holds enough matches for a weekend and isn't too bulky.
It also joins the list of things I've tried to fit in a 12 bore that isn't a real cartridge! Other items include Opinel knives, packets of polos etc!
Hope the above helps,
Dave