by anthony kashiwsky March 08, 2021
The author’s 300 AAC Blackout rifle, set up for night hunting with a digital IR night vision scope, eats a steady diet of handloads loaded in converted 300 Blk cases.
The 300 AAC Blackout (300 Blk) has been around for a minute and isn’t really considered a newcomer cartridge anymore; however, its popularity continues to rise as it becomes a more well known cartridge. It is arguably the best, most efficient cartridge out there for the now-very-popular AR pistols. Additionally, with the development of expanding subsonic and supersonic bullets designed specifically for the cartridge, it’s become popular as a hunting round for deer-sized game as well. The only problem nowadays is ammo is incredibly hard to come by. In the 2021 ammo shortage, 300 Blk seems to be even more scarce than high-demand rounds like 5.56 NATO and 9x19mm. Even 300 Blk brass and components are nearly impossible to get. However, for the 300 Blk shooter that is also set up for hand loading, there is one solution to get your stock of 300 Blk ammo back up: forming cases from 5.56/223 brass.
Forming 300 Blk brass from its parent cartridge is fairly well known, but to a novice, it might be a little intimidating. Fear not, for anyone with a reloading press, dies, and a few common reloading tools can make perfectly usable 300 Blk brass essentially for free. It just requires a little time and know-how. Hopefully, you’ve stockpiled the other components before they became scarce. Bullets like Hornady’s 125 grain FMJ still seem to be widely available, and you can find powder periodically at many places. Hopefully you have some small rifle primers, or don’t mind paying other people for them. Brass however, has been the one thing that’s been out of stock for a long time. Let’s dive into how to find usable brass and turn it into cases you can hand load.
Your first obstacle is to find usable, once-fired 5.56 NATO or 223 Remington brass. Now where are you going to find that? It may take some resourcefulness, but it is out there. Ask friends who shoot if they save brass. Many people do with intentions of “loading it one day” but never do, and might give it to you or part with it for a quick buck. You can also find people selling once fired brass online. However, you can almost always find free, once-fired brass at one place: a public shooting range. No matter what the ammo market is like, you can almost always find someone shooting their AR-15 at the range, piling up their brass, and dumping it in the brass collection barrels. Look for these people, and ask them if you can have their brass. Even offer to pick it up for them. In my experience, they’ll often let me have it, and some have even given me a confused look as to wonder what I’m even going to do with it. Oh those poor, naïve souls. They know not what a precious resource they waste.
Once you have a pile of brass, there are a number of steps to turn it into 300 Blk brass. The first step is to sort your brass. Not all 5.56/223 brass is suitable for 300 Blk cases. The issue here is case wall thickness in what will become the neck of the new round. You will be cutting off the brass right below the shoulder of the 5.56 case, and that area will become the neck of the 300 Blk case when you form it in the resizing die. If the new neck is too thick, a loaded round will jam in the neck portion of the chamber because the diameter of the neck of the loaded round will be larger than the chamber neck once a bullet is loaded in the case. You are looking for a case wall thickness of .010-.012” ideally. Thicker necks can be turned down if you have the proper tool, but for most it is easier to just sort the brass and only use that which is well suited for converting to 300 Blk. A fairly comprehensive list of good and bad head stamps or brands of brass to use exists at the 300BlkTalk forum.
As mentioned in the thread, a good rule of thumb is most American-made brass is good to go, while foreign-made brass can be too thick much of the time. If you are unsure whether the brass you have is good or not, you can measure the case thickness with calipers or a micrometer. More on that later.
Once you have your brass sorted, converting it to 300 Blk brass is fairly straight forward. If you already hand load, you likely have most of the required tools. You will need:
A few essential-but-required tools you might not have on your reloading bench include:
The next step in converting your 5.56 brass to 300 Blk is to cut the brass off right below the shoulder using a mini miter saw. These saws can be readily found at your local big hardware stores or online. Simply secure a case in the jaws of the saw, and make your cut right below the corner where the case body starts to taper for the shoulder. Cutting at this location actually creates a case that is a bit longer than it needs to be, which will require a few extra turns on the case trimmer further down the road, but I like to cut here for a few reasons: 1) The difference between cutting just below the shoulder junction and making a case that is too short (essentially ruining the case and wasting brass) is not that much. The shoulder junction gives me an easy, repeatable reference point while cutting so I don’t mess up cases. 2) Because of the case’s body taper, the cut of the case is actually made at a very slight angle, i.e. the new case mouth will not be square. Cutting cases and leaving a little extra length gives you plenty of material to square the case mouth when trimming later. (NOTE: a few small online manufacturers make jigs specifically for these saws that holds a 5.56 case at the ideal position and orientation for cutting that solves the two problems mentioned above. While I currently don’t use one, they are highly recommended for doing large volumes of brass conversions. The following link will take you to a jig that is very popular with 300 Blk reloaders:
Cutting the case right below the body/shoulder junction leaves plenty of case to square the new case mouth without risk of cutting it too short.
Next, use a case mouth deburring and chamfering tool to, you guessed it, debur and chamfer the new case mouth. This will make the next step go smoother and help ensure that you don’t need too much pressure to form the new case or that a case will not get stuck on the expander ball of the die. Trust me, a stuck case in a die is an exercise in frustration that you want to avoid, and the stuck case can usually only be cleared with special tools that won't ruin your die as you attempt to dislodge the case. It is here, after deburring and chamfering, that you can now check the case wall thickness of any brass you find questionable. Use a pair of calipers or micrometer to check that the case wall is not overly thick and will cause chambering issues (I prefer mine to be under .013”)
Notice the clean edge on the new case mouth after it has been deburred (left) opposed to the freshly cut case
Now we form the new case geometry. Thoroughly lube the cases as you would for any resizing operation, making sure that the inside of the case mouth is also lubed. Then simply run the cut and deburred cases though your resizing/decapping die as you would once fired 300 Blk brass. The case is not being necked down very much, so it does not take much force to form the new neck and shoulder. If you have to use an excessive amount of force, stop and evaluate the situation. Check that the cases are lubed enough and also check that the expander ball is down far enough, out of the neck portion of the die. These cases are short, only 35mm, so there is not much room between the neck and expander ball in a die. If your cases are getting stuck, that is a likely culprit. Move the expander ball down and you should be OK.
(Left) Prepared brass case before resizing. (Right) Notice the newly formed neck and shoulder after the case has been run through the 300 Blk resizing die.
What you will end up with after sizing is a formed 300 Blk case with an overly long neck. The next step will be to use a case trimmer to trim the cases to the SAAMI specified case length. The trimming operation leaves a bur on the inside and outside of the case mouth, so guess what…debur and chamfer the case mouth again. Now you can check to make sure that the case is the proper length. I like to trim my cases a little longer than the SAAMI minimum length. That way I make sure that I don’t trim a case too short, but it also gives the case plenty of room to stretch before it is out of spec. You can also turn the necks to acceptable thickness here if you are using foreign brass.
Trimming the case not only brings the case length into SAAMI spec, but also squares the case mouth from the cutting operation. Make sure to debur and chamfer the case mouth again after trimming.
The final step is not always necessary, but one that solves a problem you will undoubtedly encounter if you are converting 5.56 brass. The last step is to swage and uniform the primer pocket using a primer pocket swaging tool in your reloading press. The vast majority of mil-spec 5.56 ammo has the primer staked into the case, i.e. tabs are actually swaged into the brass around the primer pocket to ensure the primer doesn’t fall out of the case under extreme battlefield conditions. I have even seen primer staking on factory-loaded 300 Blk, likely because the same tooling was used to prime the cases as it is for 5.56. The spent primer can be pushed out of the case during decapping, but unless those tabs are removed, a new primer won’t be able to be seated in the case. The primer pocket swaging tool will swage, or smash those tabs out of the way and uniform the primer pocket. Nowadays, I do this whether I’m converting mil-spec brass or not. I find just having uniform primer pockets makes the priming operation smoother, as some head stamps may not have staking, but do have tight pockets without a nice chamfer, which makes seating primers difficult. It’s an easy step that doesn’t take much extra time.
(Left) The primer pocket swage (i.e. smashing metal into shape) will flatten out the primer staking and widen the primer pocket mouth for easier priming. (Right) A finished 300 Blk case converted from a Winchester 223 Rem. case
That’s it! Now you have 300 Blk brass that you can clean and load as you would normal once-fired brass. Your friends will wonder where the hell you are finding 300 Blk ammo, and how you are getting so much. You just keep that to yourself, and keep offering to dispose of their 5.56 brass for them, like a good friend should.
The stages of transformation from 223 Rem to 300 Blk (left to right): loaded 223 Rem. round, empty 223 Rem. case, cut, deburred and chamfered 223 Rem case, formed and trimmed 300 Blk case, loaded 300 Blk round
A final note: Don’t be disappointed if you don’t get sub-MOA, match-grade accuracy with your newly formed brass. If you are converting a bunch of range brass of mixed headstamps, the inconsistencies in tolerances of all the different brass will invariably lead to slight inconsistencies in accuracy. You can reduce these inconsistencies by making sure your conversion process is as consistent as possible. Trim all the cases to the exact same length, uniform all the primer pockets, and load them with meticulous consistency. It also helps to have all the same head stamped brass if you can. I have shot a lot of converted brass lately, and my load with Hornady 125 grain FMJ will shoot MOA or slightly over, and sometimes under, at 100 yards out of my 16” Radical Firearms/Palmetto State AR15. That’s plenty good to take deer, pigs, or coyotes in hunting scenarios, or ring steel at a few hundred yards. Don’t let the conversion process intimidate you. You’ll always have ammo on hand, and that feeling of resourcefulness in tough times is almost enough reward in itself to put in the effort.
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