For all of my improved cases, I use a fast pistol powder, cotton wads on top of a piece of tissue paper, NO COW, it works every time. I have also plugged the neck with candle wax, soap, pumpkin and potato, potato is too wet, don't use it.
If the powder is fast enough and bulkier enough, it should almost fill the case, the pressures developed are NOTHING like a normal cartridge with a bullet and are quite safe. I can't see the point of fire forming with bullets unless you're hunting with them, otherwise you're just wasting barrel life, IMHO. Oh, the powder I use is Clays.