HD Tactical

300 blackout cases forming from 5.56 military-neck thickness?

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  • FrommerStop

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    I have a 5 gal bucket of military brass that I have not yet sorted, but I am sure some of them will be too thick for proper function and safety if used as is for 300 blackout brass. People sort them according to head markings. Sounds like to me the loaded rounds should be checked any way.
    Question: People claim that it is not practical to reduce the case neck wall thickness. Is it that hard to use a .308 reamer on a sized case that has not yet had the neck expander through it to get the right wall thickness. Other than the work is there any other downside to reaming overly thick case neck walls. I have ordered a tube micrometer to measure case neck thickness

    Use only cases with the proper wall thickness. Take a look at the chamber drawing in Figure 1. Notice that the diameter of the chamber at the location of the case neck is 0.3350". Since the bullet has a maximum diameter of 0.309", that leaves only 0.026" of total space available for the brass. There should be about 0.002" of total clearance around the neck for reliable feeding, so the maximum case wall thickness is about 0.012". Keep in mind that 0.012" leaves almost no room for irregularities in case concentricity, so in reality you're better off with cases that are slightly thinner than this; a 0.0110" to 0.0115" wall thickness is just about perfect.

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    300 black chamber.png
     

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    Daezee

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    I've never used a neck turner or reamer and don't intend to, it sounding like too much work to me. I've certainly had problems with formed cases and too thick of necks from foreign brass when I was learning. My rule of thumb, based upon past experiences, is to only form Blackout from U.S. made .223/5.56 brass. No further problems. I even shoot pulled 7.62x39 bullets in the Blackout (with cases made from U.S. made brass) with no problems. I agree with the chart you posted.

    Of course your mileage my vary, depending upon the dimensions of the chamber of your Blackout. I've got 2 Blackout bbls that seem to have a tighter chamber than all my other Blackout bbls.
     

    FrommerStop

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    My brass is older US GI military brass that I got from the estate of an NRA competitive shooter. In past there was ammo that was given or sold cheaply to people in competitive shooting activities and this fellow collected everything. I have a lot of it and hopefully it will be good. apparently the annealing extent or extension from the mouth of the down to the area that is sized down for the new neck varies. Some of the american brass is said to be better for this leading to less neck splitting.
    If you can fire .310 plus AK bullets in your cases it sounds like your neck thickness is not excessive. And if the gun handled the sight increase in pressure that is also good. The 110 gr m1 carbine bullets should do well also.

    Thanks for information
     

    Jester896

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    I picked up 3K bent neck primed LC pull downs to cut mine from. They seem to work just fine in the 2 I have tried them in.
     

    FrommerStop

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    Right I am gathering the information and equipment. Just picked up a 2 inch mini-chopsaw at harbor freight that is recommended by some. other things are on the way. I have an old set of dies for the 300 whisper that may or may not work. The one place the the whisper exceeds 300 black out dimensions is in length between base to shoulder
    I likely will shorten the sizer a few 1000's if needed on the base or buy a new die. Really depends on the chamber of the uppers that I end up getting.

    Base to shoulder
    0.8664 for the blackout vs .869 whisper
    300 whispter vs blackout.jpg
     

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    Jester896

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    I have a Mo's gauge if you need to borrow it to set things up.

    Think I bought my jig on eBay....It replaces the vise on the saw...uses a ball detent.
     
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    Daezee

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    I use a Redding 300/221 die set. I found for forming I had to have the sizing die firm against the shell holder. For resizing I had to unscrew the die out one half turn from shell holder contact or the die would set the shoulder back too much letting the case go too deep into the chamber. YMMV.
     

    Jester896

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    after i chopped them all we presized/formed slightly with a Dillon .30cal trim die and trimmer... then I sized with the set Daezee mentioned set up with the Mo's
     

    Rebel_Rider1969

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    Wow. I don't think I'll be trying this anytime soon. I think I might have understood about 1/2 of it. I saw a jig on Ebay for 12 and thought hey I could make some out of 5.56 brass. Uh, no. Later maybe.
     

    FLT

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    It sounds more complicated than it is. You can also buy cases that are ready to load pretty reasonable. I’ve bought from these guys and find their cases to be ready to load and of good quality.

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