Originally Posted by meat head
Thanks BROZ,and KIRBY, for the info had heard of it but didn't know how to do it.
You can use pretty much any handgun powder that has a burn rate of Unique or faster. The faster the burn rate, the more careful you have to be to make sure your not producing to sharp of a pressure spike. A proper fireforming load should literally fall out of a chamber when fired in a clean chamber. If you have any resistance, your either far over pressured or the chamber is dirty.
Again, the more consistant you can get in compressing the powder/cornmeal column, the more consistant your forming will be.
Also, you HAVE to have at the most zero headspace and preferably a chamber set up for around 0.004" crush fit. If the headspace is much over zero, the firing pin will drive the case forward in the chamber until the chamber shoulder stops the case and then on ignition the shoulder will blow out but there will not be enough pressure to force the head of the case back against the bolt to properly fill the fireformed case to fill the chamber.
When this happens, you will not get really good consistency until you fireform the load again under high pressure live fire loads.
As long as the chamber is set up correctly, the chamber will hold the case solidly against the bolt while firing so you will fill the chamber properly and should get excellent results from the start with your newly formed brass.
If you see any primer set back out of the case head, that tells you that your headspace is not set up in the chamber OR in the brass your necking down. There are ways to correct this. If your necking down a case, for example in my 7mm AM, you take a 338 Lapua and neck it down to 7mm so you can position your secondary case shoulder wherever you want to get that perfect 4 thou crush fit.
With a round like the Improved 338 Lapua, I have seen a few lots of even Lapua brand brass that were a bit short in headspace. There is a simple fix. the Lapua brass has a very good anneal on their necks and shoulders. Get a 35 cal expander, neck your brass up to 35 cal and then neck back down repositioning the shoulder for the correct crush fit. This will eliminate any case wall thinning which is generally not a problem with the Lapua case but other cases are not nearly as strong and it can be critical to get the fireforming correct or you will give up some brass life.