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Converting LC 5.56 to 300 B/O - Thoughts on my process steps


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First off thanks to this forum and its members for providing the link to the Zep Reloading jig for cutting down 223/5.56. brass to 300 B/O. My jig arrived a week ago and it was super simple to set up and works great.   Now to my question to the forum braintrust. 

Here is the process I think I want to follow for my conversion. I am only going to be converting 600 pieces of brass at this time because affordable LC 5.56 brass is hard to find right now and these 600 pieces should last me until the high brass prices subside (I hope).  

Please tell me if I am off track in any way or if there is a better way.  Also a very important point...My Dillon 600 Stand alone Swage machine is set up for small primer and has the 223/5.56 case holder on it....


Processing steps -In order on my single stage press (Not going to break down the Dillon 650 since I am running it on 9mm pistol rounds right now or should I - since I have extra tool heads available)....but keep in mind I cannot/will not swage on my 650 (Do not want to void machine warranty).


1 - De-Prime with my FW Arms Primer Popper die

2 - Swage on the Dillon 600 Swage Machine since it is set up and 223/5.56 cases align nicely with a small necked brass like the 223/5.56

3 - Cut the brass on my Zep Jig using my HF 2” chop saw

4 - Lube and Size

5 - Tumble in corn cob media to get lube off

6 -Blow out cases with air compressor hand wand to insure primer hole and cases have no media “stuck”

7 - Load 


Feedback/Thoughts/Comments/ Recommendations


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Size the brass first before trimming: way easier to work with that.  Reverse 3 and 4.   Did about 100 rounds cut first and size not fun: Did about 1000 rounds size first then cut.  Remove decapping pin from your die before resizing.  

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Wouldn't it be better to do a rough cut with the chop saw and then cut to finished length with a Wilson trimmer or something similar?  I think you'll have a huge variation in case length using just the chop saw.  Also, although not necessary, you might consider some type of case processing setup to uniform the primer pockets and debur the case necks.  Here's how I would do it.

  1. dry tumble
  2. deprime
  3. swage
  4. clean primer pockets
  5. rough cut on chop saw
  6. size
  7. finish trim OAL
  8. debur
  9. wet tumble
  10. load
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Broom - I thought (correct me if I am wrong) that once a piece of brass is swaged it did not have to be swaged again...Is this wrong?  If so please let me know....


Danny - Thanks Sir - This is the reason for my post - to learn from others experience....


Itdmstr - Thank you for reminding me that I left out a step - Debur - I have a Giraud Tri-Way trimmer that I use to debur and insure trim length is consistent.....which would be my 4A process....and I appreciate and like your process steps....


Thanks guys again...Mark


Edited by Sigarmsp226
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The fastest and easiest way I have found: size without decapping removed, cut, RCBS trimmer to exact length and resize again with decapping pin installed.   You also have to work on the primer pocket.   It takes time to do this now:  I also turn the necks.   Shot 50 rounds yesterday and turning the neck really helped.    It's a process and can be painful.   30 years ago when we made 7 TCU from 223 it was easier because we didn't use 5.55 brass only 223; that gave us more uniformity.   300 blackout is a good round: I use a 168 grain cast bullet with 3031.   It's works out to 200 yards easy.

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  • 2 months later...
On 2/21/2021 at 1:04 PM, Sigarmsp226 said:

Broom - I thought (correct me if I am wrong) that once a piece of brass is swaged it did not have to be swaged again...Is this wrong?  If so please let me know

You are correct.  Once swaged you don't have to do it again. 

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