Rimless and rebated cases are easy to fireform for Ackley chambers if the headspace is correctly set as described earlier in this chapter. You can fire factory ammunition for the parent caliber in the chamber with no ill effects. The result will be velocity slightly reduced as compared to firing the same load in a standard chamber. This loss of velocity is primarily as a result of energy being used to form the brass, secondarily because the larger volume of the improved chamber requires more powder to get the same velocity.
Rimmed or belted designs headspace on the rim or belt respectively so you can fire factory ammo and in most cases will produce good results most of the time — even though the shoulder is often blown forward. One exception would be using old fired brass: often it is too brittle and will split during fireforming. So using new unfired cases is recommended for fireforming.
If you happen to have a wildcat that requires a large amount of forming, firing factory ammo will sometimes cause an unacceptable loss of brass, resulting in split cases. This happens because the cases expand so quickly that if there is a flaw or weak spot in the brass it will pop like an overfilled balloon. One example of this would be the .219 Zipper Ackley Improved. If you experience this there are two possible solutions. Try annealing the neck and shoulder down the body to as much as half way along the body. Make sure you do not anneal the case head or the thick web area just above the head. Annealing the head will greatly reduce the case’s ability to handle pressure. Once annealed, load the cases as normal and try fireforming a few. Initially don’t load more cases than you are willing to pull bullets from.
Fireform using corn meal or “Cream of Wheat.” George Nonte, in Home Guide to Cartridge Conversions tells how to develop fireforming loads using no bullet. In Custom Cartridges Ken Howell expanded on the concept.
First insert a spent primer (for safety) in an unformed case. Fill the case with Bullseye® pistol powder to the top of the neck (DANGER, never to be fired). Then pour this charge into the pan of your powder scale and weigh the charge. Divide that amount by 10, so that 1/10 of the total volume will become your starting load. Now take that 10 percent load for your case and put it in your normally primed case. It may take some testing to get a load that fully forms the case. Tear a single sheet of toilet paper in quarters, insert one quarter as a wad over the powder charge. Now fill the rest of the case with your inert filler (corn meal, or fine ground hot cereal, etc., dry of course). You will want to place a wad of some sort over the cereal so it does not spill. Bees wax, bullet lube, or toilet paper work, it must be light enough so that the wad can be blown down the barrel without doing any damage.
You are ready to fireform a case. Keep in mind that even without a bullet these loads could be deadly, so use all normal gun safety practices. Pointing in a safe direction fire the first load. If it is not fully formed, bump your 10 percent load by one-half grain at a time until you get a fully formed case. It is possible to generate dangerous pressures if you use too much powder in these inert filler loads, so be careful. The interesting thing about this method is that it will often form cases without any loss to ruptures when a factory load will cause ruptures.
L.R. Wallack wrote this method up for American Rifleman. After describing the method he said, “I then did 10 cases with this load with no splits and all formed nicely. Such success has been practically unheard of, as anyone who has formed cases for this wildcat well knows. I have no hesitation, therefore, in recommending the method.”
Editor's Note: This article is an excerpt from P.O. Ackley: America’s Gunsmith.