Here's my procedure. For starters, not all 5.56 / .223 Rem brass is suitable for conversion. Ideally, the neck wall thickness will be 0.010" to 0.011", and suitable wall thicknesses can go up to 0.013". At 0.014" and above, however, the finished cartridge diameter at the neck may be too thick to chamber. Fortunately, there's a pretty comprehensive list of brass types and their wall thicknesses over on another forum. Keep in mind, this is just an informal list on the internet, so don't take it for gospel. You'll need to verify the accuracy on your own, but it's a great starting point. If you find it helpful, it may be a good idea to take screenshots of that page to save on your computer. I've had good results with FC brass, so I'll be using that below.
https://www.300blktalk.com/forum/viewtopic.php?t=88599
Onto the brass conversion. There are different ways to do this, but this is what works for me. First, you'll need a mini cut-off saw to trim the neck off the 5.56 brass. I bought mine from Harbor Freight. They currently sell for $40.
https://www.harborfreight.com/2-in-mini-benchtop-cut-off-saw-62136.html
Next, you'll need a jig that mounts to the saw to hold your brass in place to cut it off. There are a number of different jigs out there that achieve the same thing. This one is from Zep Reloading, and costs $30.
https://www.zepreloading.com/store/c1/Featured_Products.html
The way the jig works is that you slip an empty case into the jig, followed by another case to push the neck of the previous one out the other side. There's a spring loaded detent that captures the case at the rim and holds it securely to be cut. After cutting one piece of brass, you simply push another in from the left to eject the newly cut case and lock the next one in line. I can trim 100 cases in about 5 or 6 minutes. Some people rig up chutes and baffles to help direct the ejected brass, but I haven't bothered with that. At most, I may lay a narrow board across the edge of my work bench so the brass doesn't roll off.
A lot of people run their freshly cut brass through their regular full length sizing die to reform the neck into .300 Blackout. I found, however, that requires a lot of muscle and results in tons of stuck cases and frustration. After a bunch of research, I discovered form and trim dies. I had reloaded for many years, yet had never known of these. They essentially squeeze down the straight walls of the cases you just cut to form the .300 Blackout bottleneck. For the trim function (which I don't use), you can screw the die down onto a case in a raised shell plate and then file the protruding brass flush with the die to its correct length. This little die makes it world's easier to form your brass and saves much frustration over stuck cases in a regular sizing die. This Redding version is $53 at Midway.
https://www.midwayusa.com/product/101622015
The form and trim die is normally used with an extended shell holder on a single stage press. Since there's no option for that on my Dillon press, I need to mount the die on the underside of the toolhead so that it's low enough to form the brass.
I use a Wilson case gage to set the form die.
https://www.creedmoorsports.com/product/wilson-case-gage/case-preparation
After this step, I still run the brass through my regular full length die for final sizing, but it is MUCH easier having been formed first. If you've set your brass jig correctly in your mini cut-off saw, there should be little length trimming to do. I use my Wilson Case Trimmer for that, and then bevel and chamfer the mouth.
https://www.creedmoorsports.com/pro...h-micro-stop-stand-and-clamp/case-preparation
Finished loads. These have my hand cast and powder coated subs. Since you've cut the 5.56 brass near the bottom of the annealed portion of the case, some people prefer to anneal their newly formed Blackout brass to avoid split necks and perhaps create more uniform neck tension. So far, I haven't found a significant number a split necks after being fired multiple times, and since we're dealing with short range Blackout loads instead of target loads, the juice just isn't worth the squeeze with annealing, IMO, especially if you don't already have an annealing setup. Well, that's it. Hopefully this helps.
