You make the cases out of 223's. It is a 223 shooting 6mm bullets. When forming brass, I always suggest to start with virgin brass, but you can always anneal with once fired brass (military). Just make sure you are using the same head stamp for your whole batch. You run the 223 brass into a 6 TCU die and it will expand the neck. It expands it from .223 or so to .242 or so; so it is not expanding a bunch. Then load them up for development. Start low and work your way up. I personally would not bother with fire forming, they should headpace OK right off the bat, if not, then you will need to adjust your dies to fit your chamber. Good Luck and Good Shooting