Converting 223/5.56 cases to 300AAC Blackout

Since I received my 300 AAC die set already I started converting some cases as I am still waiting for parts to arrive to finish the rifle build.

I've tried various ways of converting and found this to be the easiest way so far.

1) start with cleaned/tumbled 223/5.56 cases

2) cut off the case directly at or just above the case shoulder, not below it. One little oops will make the case too short after sizing. I used a dremel cut off tool for this, it was the quickest way.

3) Deburr inside and outside of the case where you made the cut to limit wear and tear on the sizing die.

4) Lube the case as you normally would for sizing and resize/convert the case. It takes very little effort to resize these cases to 300 BO. I make two passes...the first pass, then rotate the case 180deg and make a second pass, just to be sure it is formed as even as possible.

5) Trim the cases to the SAAMI specs of 1.368" then deburr and chamfer as usual.

6) Anneal the cases. This step could be done at any time in the order of steps. before cutting, before sizing or after trimming...it doesn't really matter and honestly, as easy as it is to convert these, it may not even be necessary at all. You could even anneal the cases after fire forming if you prefer.

7) load them up with a modest charge and cheap bullets for fire forming. There are other ways of fire forming that doesn't require the waste of perfectly good bullets, but since I'll be breaking in a new barrel, I figured I might as well be breaking it in while fire forming my newly converted cases.

I have 50 rounds loaded up so far and ready. I have them charged with Lil Gun powder and topped with 125gr flat based, soft point bullets. They fall a little short on the OAL compared to most other Blackout rounds, so I Might have feeding problems with these...but since it isn't a Semi-auto and I won't be using these bullets except for the initial fire forming...it's really no big deal.

I'll try to get some pics and maybe even a vid....for now, I'm itching to get to the build.

A quick pic...

From left to right: Unmolested 223 case, trimmed down 223 case, sized to 300 BO, finish trimmed, 125gr SP dummy, 125gr Nosler Ballistic tip dummy

hawcer wrote:2) cut off the case directly at or just above the case shoulder, not below it. One little oops will make the case too short after sizing. I used a dremel cut off tool for this, it was the quickest way.

If I remember correctly osomebody on this forum said that a cut-off saw was good to use for that too.

EdgarEg wrote:

hawcer wrote:2) cut off the case directly at or just above the case shoulder, not below it. One little oops will make the case too short after sizing. I used a dremel cut off tool for this, it was the quickest way.

If I remember correctly osomebody on this forum said that a cut-off saw was good to use for that too.

Anything that will cut brass will work...the best being a mini cut-off saw as you mentioned, a dremel with cutting disk as I used, a small tubing cutter as I have used before but misplaced somewhere or just trim the case down to size all the way using a case trimmer.

hawcer wrote:

EdgarEg wrote:

hawcer wrote:2) cut off the case directly at or just above the case shoulder, not below it. One little oops will make the case too short after sizing. I used a dremel cut off tool for this, it was the quickest way.

If I remember correctly osomebody on this forum said that a cut-off saw was good to use for that too.

Anything that will cut brass will work...the best being a mini cut-off saw as you mentioned, a dremel with cutting disk as I used, a small tubing cutter as I have used before but misplaced somewhere or just trim the case down to size all the way using a case trimmer.

I'm sure that somebody is gonna finger-out a 2-piece jig that can hold 10 or 20 pieces of brass ,and just slide it under a mill

I'm sure that somebody is gonna finger-out a 2-piece jig that can hold 10 or 20 pieces of brass ,and just slide it under a mill

Maybe a mini radial arm saw with a metal cutting blade? Like used in woodshop, but on a much smaller scale. Just drop it down and "zing" through 20 cases at a time,LOL.

Nice looking rounds Hawcer. What twist rate is your .300 barrel? I formed my 7.62x40WT case in a very simalar way. These are case length of 1.552. I have my barrel broke in and am working on loads for my March Hog hunt in Okla.

Nicnac wrote:Nice looking rounds Hawcer. What twist rate is your .300 barrel? I formed my 7.62x40WT case in a very simalar way. These are case length of 1.552. I have my barrel broke in and am working on loads for my March Hog hunt in Okla.

The Remington 700 I have on the way comes from the factory with a 1-7 twist barrel. It's optimized for subsonic 200+ grain bullet stabilization, but also works very well with lighter supersonic bullets.

Nice, look forward to hearing more about it. Any major plans with the rig when done? targets hunting?

Nicnac wrote:Nice, look forward to hearing more about it. Any major plans with the rig when done? targets hunting?

As of now, it's just going to be a fun gun...targets and such. Maybe some hunting down the road. Indiana is working on getting sound suppressors legalized for hunting...the Remmy 700 AAC Blackout just happens to come with the barrel pre-threaded for just such a thing. Only time will tell

Looking goog hawcer. I haven't been around the 300BO yet but the ballistics that I've seen on them look pretty good. Hope the build goes good for you.

Cool project. The dremel is definitely the way to go. I watched one guy on youtube use his brass trimmer chucked up to a drill to get the brass to length. It took him like a minute to get it shaved down to size. LOL