Forming Your Own Cartridge Cases

In this case, forming .300 Blackout cases from .223 Remington/5.56x45mm.

Why form your own cases? Cost, availability, and custom-fitting are all right up there, but the main reason is that it's how a lot of cartridges came to be:

• After WW2, when lots of troops brought back 8mm Mauser rifles, there was little to nothing in the way of 8mm brass to be found, but you could take .30-06 cases and form them to make usable 8mm cases. 
• The .270 Winchester: someone* thought "There are some really good 7mm bullets out there, very ballistically-efficient; if you necked-down a .30-06 case, wonder how that would work?" It worked very well, and still does 91 years later. 
• Another one is the .300 Blackout, also called the .300 AAC Blackout and .300 Whisper. Some people wanted to create a cartridge that would give 7.62x39 ballistics in a cartridge that would work with a standard AR-15 bolt and magazine, and succeeded. It's actually more versatile, as you can use bullets from 110 to 220 grains, both supersonic (in the lighter bullets) to subsonic, and the latter works very well with a suppressor. It's also one of the easiest to form, which is why I'm using it as my example.

.300 Blackout from .223 Remington
The .300 Blackout was originally made by taking .223 or 5.56x45mm military cases, shortening them, and forming a neck to hold a .308-diameter bullet.  A lot of the commercially-available cases are still made by forming them from once-fired military brass, and it's easy to do yourself.

1) Start with some once-fired cases
Clean them up to get rid of any propellant fouling, dirt, and anything else they might have picked up (I've picked up cases at the range and found a spider inside).

2) Cut them off to appropriate length
The maximum length listed for these is 1.368 inches; since the forming process isn't going to stretch or shorten them much, I cut these off right at 1.370". You can use a hacksaw with a very fine-tooth blade (which is a bloody pain), a jeweler's saw(the same), a rotary tool with a cutoff wheel (much better), or -- the best by far, especially if you're going to make a bunch of them -- this cutoff saw from Harbor Freight. This is what I used.

3) Make a jig

Or a block, or something that'll allow you to cut to a consistent length. There are jigs for sale on eBay, but I did the cheap route and made this:

It's nothing more than a piece of 1/16" thick, 1/4" wide steel strap.

1. Bend one end to a right angle
2. Mark and cut a case
3. Use that to adjust the strap in the vise
4. Mark the spot to bend
5. Bend that end. I'd suggest using a vise and a hammer, as you want a nice, sharp angle  for the bend. 
6. Make sure the end will clear the blade; if need be, file or grind it thinner.
   
   
7. Cut a case using the jig and measure it.
   
   
   
   
   
8. If the case is too long (remember, every bit of too-long will have to be trimmed off later) you'll have to straighten the bend and make the long leg a bit shorter.  
9. If it's a bit too short, that's easier: take a straight- or cross-peen hammer and a flat surface (your vise probably has one) and strike a couple of light blows on the flat of the long leg to stretch it.
10. Cut another case, measure, and continue until it's the length you choose.

4) Cut your cartridge case

After that you can put the jig in, push in a case, tighten the vise, cut, and repeat.

It won't take long to do a bunch of them.

You're left with these.

5) Clean the cut
Take a deburring tool and clean the burrs from the cutting off of the inside and outside of the case mouth.

Lube the cases, inside and out. If you're using a spray lube, you can make sure some is sprayed into the mouth; otherwise you'll have to use something like a q-tip to put some on the inside.

6) Resize
Adjust your .300 Blackout resizing/de-priming die so that, when the ram is all the way up, the mouth of the die touches just the case holder.

Insert a case, and run it all the way in.  That'll form the case to Blackout dimensions, including forming the shoulder and neck; as you reverse the lever to pull the case out, the expander ball will pull out and make sure the neck has the right inside diameter, which is why you want to make sure you have some lube inside the neck area.

7) Trim the cases to length
Lee makes a simple case trimmer (it works with this universal base and cutter), or you can use a Lyman, or RCBS, or any of the others available; this one's a Lyman I've used for years.

8) De-crimp
If it's a military casing, it's a good idea to take a suitable tool and get rid of any primer crimp; lots of military cases have the primer crimped in to make absolutely certain it can't shift out of position under the stresses of lots of travel, or being rattled around in a magazine or belt while being carried around and fired in automatic weapons. The crimp can make it difficult to near-impossible to seat a new primer, so do it.

You now have a case ready to be loaded

Lots of cases really are that easy to form for another cartridge: take the parent case, trim it to length, run it into the sizer die. Some require more steps, and some you don't want to mess with unless you have to/really want to.  (The worst case I've ever formed was making .30 Nagant from .223 cases.  A real PITA, and I'm very glad you can just buy reloadable cases now.)

Cartridge forming can be involved, but it gives you a way to make less-available cases from something you can easily get.

* One gun writer wrote of forming his own .270 cases from .30-06 because it gave a thicker wall in the neck; he could use a case neck turning tool to get the exact case neck diameter that fit his rifle best.