Cartridge Conversion

Returning to my series on reloading, another concept in this area is modifying one cartridge case into another chambering, perhaps even a brand new one.

If the result is a brand new case design, it’s called wildcatting. Many commercial cartridges we know today started their lives as wildcats, including the .22-250 Remington, 7 mm-08 Remington, .300 Blackout, and others. In some instances, the wildcat cartridge was developed in cooperation with commercial industry, such as the .357 Magnum, .44 Magnum, .221 Fireball, and more.

The conversion process can be fairly simple, or it can be considerably more complex. It may involve necking the parent case up or down for a different diameter projectile, moving the shoulder forward or back, changing shoulder angle, adjusting case taper, trimming case length, modifying rim dimensions, or any combination of these steps. A good general guide can be found here.

L-R: Original .223 case, resized case, cut and
deburred .300 Blackout case (streaks are from case lube)

I’ll start with one of the simpler conversions, .223 Remington* to .300 Blackout. 

1) Sort, deprime, and clean the brass.

2) Resize the case. To start this step, I apply a dab of case lube to the first case, then run it into a .300 Blackout sizing die.  This can take some force, so I use an O-Frame press as they’re stronger. 

Not every case needs to be lubed; I’ve found I only need to apply it every three to five cases with carbide dies.

3) Once the cases have been run through the .300 Blackout dies, they need to be rough cut slightly oversize. I use a small chop saw I bought at Harbor Freight.  There are jigs available to hold and position the cases on the saw bed (like this one or this one), but I made my own out of some scrap wood and a couple of brass screws. More recently, I 3-D printed a trim jig for more precision.

4) After the cases have been rough cut, they are trimmed to final length. Again, there are several different tools available for this step. I like the Lee Precision trimmer that can be used with my drill press, and each cartridge has its own Case Length Gauge and Shell Holder.

5) The final step at this stage is deburring and chamfering the case mouth. I use a classic RCBS hand deburring tool. Once this is done I clean the cases in a vibratory tumbler with corn cob media to remove the lubricant and any brass shavings.

6) In this particular conversion, there’s an optional step that can be done before sizing; in other conversions, it’s an absolute necessity. It’s called annealing. Brass work-hardens and becomes brittle with use, and case conversion can heavily work the brass. Annealing reduces the chance of cases splitting during the conversion process and can also increase case life as well.

For those interested, there’s a discussion in the Reloading section on AR15.com on making your own DIY Annealing machine.  I haven’t done this yet, but it’s on my list.

Once the brass is converted to .300 Blackout, it can be shot and reloaded multiple times.  If and when the case finally fails, it’s usually due to the case neck or mouth splitting due to work hardening. At that point, the brass can be discarded, or it can be trimmed and sized further to be used in .380 ACP handguns.

Converting .223 Remington to .300 Blackout is one of the more simple and straight forward case conversion. A much more involved one is converting 24 Gauge Magtech Brass Shotshells to .577/450 Martini–Henry; here is a good video of the process.

That conversion requires multiple sizing and annealing steps due to the considerably more significant changing of dimensions and therefore increased work hardening of the brass.

For other conversions, it may be necessary to redimension the case rim or extractor groove.  This is best done on a lathe as the amount of material to be removed is precise, and if the rim needs to be made thinner, material is generally removed from the inside of the rim, by which I mean the portion of the rim opposite the case head.

Obviously there’s much more to this topic then I can cover here.  If the idea of converting one cartridge case to another interests you, there are a number of resources available both in print and online. The two I reference the most are Cartridges of the World and The Handloader's Manual of Cartridge Conversions.

Case conversion is an excellent way to not only get some older firearms shooting again, but is also a great way to learn more about the family relationship between some otherwise very different cartridges.

Have fun, and safe shooting!

* 5.56mm cases can also be used, but they have two additional concerns. The first is that the primers may be crimped in place, and removing the crimping is an extra step. The other has to do with case wall thickness: since the case length is trimmed back significantly, case wall thickness may become an issue when the new case neck is formed. This can increase neck tension which can lead to increased pressures when firing.