The first 204 Ruger case I necked down to 300 Blackout was not on purpose. I discovered the first one by mistake, after the fact, and it led me down the internet rabbit hole, looking for data to make sure I wasn’t going to cause problems. I also didn’t want to have to go looking through all my reloads to make sure I didn’t have any others mixed in if I didn’t have to. I spent several hours doing research and consulting with experts on if I could convert 204 Ruger brass into 300 blackout. Here’s what I found.
About the 204 Ruger
The 204 Ruger comes from the same parent case as the 300 AAC Blackout and the 223 Remington- the .222 Remington. This gave me more hope that I could neck down the 204 Ruger brass into 300 Blackout.
Note that all the dimensions and angles of the bolt faces of the cartridges are exactly the same. Based on this information, in theory you could use any cartridge from that same family– .221, .222, .222 Magnum, .223, .204, .17 Rem, .17 Fireball.
Case Wall Thickness
Going down the internet rabbit hole, one forum recommend that I check the case wall thickness. That guy claimed that if you were above .012″ you may need to ream it down a bit. This was comforting to me. If you have to ream it down, .012″ or more should be plenty thick. I put the calipers on it and I got .013″. For comparison’s sake, I also checked some necked down 223 (.012″), necked down 556 (.012″), and some once fired factory 300 AAC Blackout (.0105″). I was initially worried about the case being too thin and having risking a failure in that way, but instead my concern may be a failure to feed due to it being too thick.
When reloading, I always to a slight ream on my case mouths if I trimmed them to debur them with the good old Lyman case prep tool, so I figured I was probably good to go. If it’d chamber without difficulty, I was going to be in business.
I Asked my Local Gunsmith
I like to get multiple sources of information when I try something new with reloading. I like to be precise, and I like to make sure I’m right before I pull the trigger. Because of this I asked a local gunsmith that works a lot with the 300 blackout. His response was that its just a necked down .223. Someone took a 5.56 and made it a 5mm. Like they didn’t want the .223 or the 17hmr, so they split the difference. If you cut the top and necked it down to 300blk, then you’re fine. You won’t even know the difference.
This gave me enough confidence to load it up and send it.
Actually Shooting the Converted 300 Blackout from 204 Ruger
While there were plenty of forums saying they had necked some 204 cartridges down, I couldn’t find anyone who had actually shot it. Either they hadn’t reported back, never shot those reloads, or had a catastrophic failure and died. At this point I had spent about 3 hours researching this one cartridge conversion. I was invested. I had to go shoot it.
I was more excited to shoot this 204 case converted to a 300 blackout than I have been for a reload in a while. I’m not sure what I was expecting, but it was kind of anticlimactic (which is probably a good thing). I’m not going to make drama where there isn’t any. Yes, it works. It shot just like the factory 300, the 223 converted stuff, the 556 converted stuff, and the once fired 300 blackout reloads. I had no issues whatsoever. Below is a picture of the once 204 Ruger cartridge converted to a 300 blackout, after being fired. There were no signs of over pressure, it cycled great, and it busted the clay pigeon I was aiming at, so it was plenty accurate.
Can I convert 204 Ruger brass into 300 blackout?
Yes, by the data and by our actual field test, you should be just fine to convert 204 Ruger brass into 300 AAC Blackout brass. To cover ourselves we should add that you should use proper loading methods and safety procedures and do your own load development, but it worked for us, and there is no indication from the specs, the data, or any other source that this wouldn’t be something you could do.
I’ve had other problems with variations of 300 blackout loads I’ve made before, but 204 brass won’t cause you any problems.
Should you cut them?
This is personal preference. 204 brass is worth more than 223 or 300 AAC Blackout. There were some internet listings of guys that are willing to trade 2:1 for the 204, trying to talk people out of converting 204 to 300blk. But, if you just have some and don’t want to deal with the hassle, yeah, go ahead. Honestly, I’ve picked up thousands of 223/556 brass and have only found one 204, so its really not worth my time to try to trade them. I’ll probably put any more 204 cartridges I find in my “uncommon” bin and not cut them in the future, especially because, like I said, I have thousands of 223/556 casings, but its always nice to know that I’m able to. It’s also nice to know that if I do it accidentally, it’s not a problem.
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