Reforming a case is literally to change a case from it’s designed caliber and profile into a different profile or caliber. If you have read some of my other articles on reloading or lead casting, you may have noticed that when describing some procedures that I will say that some are left to more experienced loaders. Once you have reloaded for a while you will get a feel for how cartridge brass behaves when it is stressed, sized, and fired. More importantly, you will also know what to look for when determining the safety of a piece of brass, and how to evaluate non-critical flaws (like excess lube induced shoulder dents), vs critical flaws like neck splits, fissures, structural folds, deep scratches, and impending case head separation lines/bands near the case head base. Once you have these skills, you can explore some of the advanced brass handling procedures.
Why reform brass? There are many reasons. In the past, sometimes it was hard to find a certain caliber, perhaps a newly imported military caliber that has no domestic ammunition source, or the other way around. You may be shooting a nice old firearm for which ammo has not been commercially available for years or decades or longer!
Case reforming can be as simple as trimming a .44 Mag case down to .44 Spl length. It could also be an easy conversion simply involving running your original case into an appropriate full length resizing die, then trimming, then fire forming, such as converting .270 into .30-06. Other times the parent cartridge may be significantly different enough that you need a series of smaller changes so as to not overstress the brass during any one function.
To make radical case change easier, some companies have made die sets that may have from 1 to 3, or 4 or more dies that each performs a small step or procedure. You can purchase special custom sets that are intended to convert one caliber into another, or you can, as mentioned above, find full length resizing dies from your parent and target caliber, plus perhaps other intermediate calibers and home brew a cartridge conversion.
While each case conversion will have its own custom die set and steps, here is an example that RCBS uses to explain a multi-step conversion of a 30-06 into a 250 savage. Run your case into the first form die to set the initial case taper. The neck and shoulder are not addressed; however, the shoulder will move as the case is being tapered. The next step would be to set the shoulder back using the next die in the set. At this point you may need to trim the neck back since as you are sizing this 30-06 down to a smaller case, the excess brass has to go somewhere, and much of it ends up as an elongated, thickened neck. Trim dies are often included so that you can use a fine-toothed hacksaw or similar to trim the neck off very close, perhaps 1/32nd above the trim die, then use a fine tooth flat file to finish removing material and squaring the mouth true to the trim die opening. The next step is to reduce the cartridges neck diameter in another die, then once again, trim down the excess brass that ‘grew’ during the sizing. At this point, the case transformation is nearly complete, and your formed brass will look almost like it’s intended target. Your final steps would be to expand the neck, then inside ream, as it will have thickened during the forming stages. At this point, a trip thru the intended cartridges full length resize die and final trim leave you with your ready to inspect, then load brass case. Throughout this process, case lube MUST be used to reduce galling and ease effort needed to cycle the brass in and out of the dies. For these very hard resizes, I prefer Imperial sizing wax as a case lube. Apply with your fingertips, just an invisible thin film is needed.
One interesting case reform I have done is converting a rifle case into a pistol case. A while back, a good friend noticed that I had a type 14 Japanese Nambu pistol. They fire the 8mm Nambu cartridge. (They are commercially available now, however, used to be un obtainable decades back when the guns first appeared on the US market). Anyway, my good friend sent me a case reforming set for the 8mm Nambu that he had no use for. (Thank you!) The set was an older RCBS, and the parent case it wanted to use to reform into 8mm Nambu was the 30 Remington. Now that almost posed a problem! Nowadays, 30 Remington cases are more valuable and harder to come by than 8mm Nambu! Not to worry though, as a little research showed that a 6.8 SPC case was based on the 30 Remington. I have plenty of 6.8 SPC brass. A little measuring showed that it would work. First step was to cut the case down, in half almost. Next, the bottleneck was formed, neck expanded, throat reamed, and then a trip thru the 8mm Nambu size die. The result was perfectly functional brass that hand cycled and loaded fine. The big difference was when you flipped the case over, one took a small pistol primer, the other a large rifle primer. (Remember, if you were loading for 8mm Nambu with a small pistol primer and started to use cases that took large rifle primers, that you would reduce your load to minimums and work back up to check for pressure issues!). A fun project, but rather labor intensive. I’m glad 8mm Nambu brass is commercially available these days!
Case reforming is a skill that at its most basic, creates options where none may otherwise exist. If you have a firearm in an obsolete cartridge, your ONLY option to fire it may be to make or acquire converted cartridge brass. Often, obsolete brass may be so expensive to obtain that the cost of the reforming die set is quickly recovered once you process a few batches of brass. These hobbies are informative and can be addicting. If you are reforming cases, then you reload. If you don’t lead cast your own projectiles, you might want to consider that as well. You can even make your own bullet lubricant! It’s a very good feeling setting at the range shooting the cartridges that you reloaded with the brass that you reformed and bullets that you hand cast and made the lubricant for.