300 Blackout case conversion problems

by Erik  

I've spent a fair amount of time troubleshooting the process of converting .223/5.56 cases to 300 blackout. Here is the setup I'm using:

My original plan was to save my many times used .223 reloads with split necks. I average about 5 reloads on a .223 case before the necks start to split. Initially, I was converting them with no issues. I had them trimmed to around 1.350" and they gauged just fine. I realized some of my newly reloaded 300 blackout rounds would not chamber correctly. The bolt appeared to be out of battery. The forward assist did not work and round was noticeably hard eject manually.

My range day was cut short when I realized about 1 of every 15 or so had this problem. I loaded over 300 rounds so I figured I would pack up and try to figure out what the issue was. I started by looking at the head stamps. While shooting, I noticed that most of them were RWS cases. I put all my loaded rounds through the EGW gauge and noticed that while they all gauged fine without bullets, some where no longer gauging and some were barely still gauging. In some cases, an additional crimp fixed the problem but that wasn't a real solution.

So I ended up finally getting to the bottom of it. The reason the RWS cases didn't work is due to the actual case thickness. I verified this by reading various forum posts and comparing them against two different types of factory 300 blackout cases using a caliper. The magic number is .334 which is the outside case diameter needed to properly seat in the chamber. We arrive at .334 by:

  1. .308 (projectile)
  2. .013 optimal case thickness. Double to .026 for each side of the projectile
  3. .013 + .308 + .013 = .334

Back the RWS case issue. I cut some more of them and noticed the case thickness was around .015 which puts the loaded rounds over the magic number of .334! Several other brands of cases proved to have the same problem. At the end of the day, the easiest way avoid having this problem is to use Lake City brass for 300 BO conversions. I used a few hundred LC cases from various years and they all worked just fine.

The only downside to using the Dillon 300 blackout forming die/trimmer is that because the blade comes down on top of the case, there is not expander ball present like a full-length re-sizing die. While it cuts and forms the shoulder, the case mouths are severely undersized. This requires another step using either a sizing die or neck-sizing die. Since I don't own a neck-sizing die I run them through the full-length re-sizing. I then verify them with a case mouth gauge. This is critical to ensuring you can properly seat bullets. If you have too much neck tension, the will be very difficult if not impossible to seat the bullets without cutting into the jackets or bending the case. With too little, the bullet can be pushed back into the case causing an unsafe condition.

IMPORTANT: Your unloaded AND loaded rounds should fall out of your case gauge. If you have to push them out front to get them out of the gauge something is wrong. A good example of this is factory new ammo. It's slides in and out with no issues. Your reloads should behave the same way.

For a list of approximate thickness, see this thread:


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