I normally use a much faster powder than normal, anda real cheap or inaccurate bullet. For instance, when fire forming the .270 Winchesterto .270 Ackley Improved, I use 22.0 grains of N-110 with some bulk 130 gr. bullets that I have from the early 70's. For the .22-250 Ackley, I use IMR 4198. Basically, just enough powder to blow the case out so the shouldermakes a nice crisp angle. Also, if you have some CCI 250 large rifle primers laying around, this is a good opportunity to shoot them up. These fire form loads can be fun for plinking, or for fall turkey hunting as well. Low recoil, low noise, minimum tissue damage.

I know some folks who have tried using Unique with some kapok and toilet tissue for this task, but sometimes you get primer setback - which sorts of sets you up for a headspace problem on subsequent firings.