AR Barrel Chamber Question??

[5.56Geo]

Is there anything in an AR barrel chamber that can contribute to head case separation of brass? I am almost sure it's the brass but I would like some technical assistance before I throw out the brass (1000+ pcs) or start barking up the wrong tree! I have had (12) twelve head case separations in the last 300 rounds. I will list the variables below to assist in forming an opinion.

Variables;

Upper:
New (300+/- rounds) factory built upper (well respected manufacture)
Chrome lined HP tested barrel, 5.56 16" Mid-length
Mil-spec. BCG(all testing done to be considered a good BCG), same brand as upper

Ammo:
Hornady 55gr projectile
IMR 4198 powder 21.7gr
C.O.A.L. = 2.217"
Brass- mixed head stamp range pick up (first time buying from this supplier).

Reloading press- Redding T7 Turret press
Dies- RCBS .223 SB black box

I am at wits ends! I strongly suspect its the brass but don't want to throw away 1000+ pieces away if I don't have to. I have not tried factory loads yet do to the fact that it happens so randomly. It happened once at the range the others out at the desert with no where to buy ammo. After the brass is tumbled and cleaned there is absolutely not indication that there may be issues with it. ...could it be the barrel chamber?

Thanks for the help.

[Onyx Z]

Ditch the SB dies first. They are useless.

Post some pics of a sized case and the separated case side by side.

[5.56Geo]

I don't have any of the cases that have the HCS other than a picture of the first one.


Here is a picture of the last HCS that jammed so bad I had to beat the charging handle to get it out. You will notice the mouth is damaged, I did that later with needle nose pliers trying to break it loose from the live round to examine further. You will notice loaded ammo from the same batch of brass, the top part of a HCS case with a live round in it and sized unloaded brass.


More unloaded sized but un trimmed brass.


Why would you get rid of the SB dies. What dies would you recommend? Thanks!

[Ryno12]

If I were you, I'd repost your question in the ammo & reloading sub forum. I know you're specially asking about a barrel chamber but the hardcore reloaders may be able to shed some light on your issue.


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[mpom]

suspect either the chamber is oversized or the brass is undersized. with an unproven (new) barrel, you can run factory ammo or check chamber with a MAX headspace gauge.
SB dies are unnecessary as regular dies will work just fine without over working brass.

[Renegade04]

I am suspecting the brass as well. What are some of the headstamps on the brass? Any idea as to how many times the brass had been used before. In most cases, with good brass, you might get three uses from it before it weakens too much to be used again. A lot of that depends on how hot the loads are. Do you have the same problems with new factory ammo?

[jstone]

Use aa paper clip or a dental scraper to check for signs of seperation. If using a paper clip, straighten it then bend one end to make a hook. Stick that in the case and run it from the case head along the body. If you feel a place where the tool hangs up you know that case is beginning to seperate.

Im on my phone right now so typing directions is not the easiest thing to do, but if you google inspecting brass for incipient case head seperation. You should get all the info you need. Then you will be able to keep the good brass and recycle the stuff that is close to seperation.

Edit: the pics of the loaded rounds do not show any signs of seperation. Like a bright shiny ring where the case is going to seperate. Get rid of the small base dies, they are not necessary. Just pick a standard set of dies. Small base is not needed for an AR unless the chamber is to tight. People that claim you will get your best reliability with them are just regurgitating nonsense. There are some cases where they are needed, but there is not many. When you set up your sising die set the die to just kiss the shell holder. Size a piece and see if it fits. If it does size a few more and make sure they fit. If it does not fit twist the die in small increments until it fits. When I say small increments I mean like less than 1/8 th of a rotation. Setting up di3s to the instructions usually will bump back to saami dimensions. Which is like .008 or more shoulder bump. Doing this repeatedly will lead to case head seperation. Your primer pockets should be toast before you get separations when using minimal shoulder bump. Just make sure it is enough to feed reliably. .003-.004 should be enough for reliable feeding if you have a way to measure it.

[lunchbox]

Was brass collected off a machine gun range or regular. I have been told MG brass can have low reload life and some times can have case separation problems. I have never used MG brass, just relaying info (that sounds plausible) as it may relate to this circumstance (I might be wrong). edit: FL resizing brass is less stressful on brass and gives more reloads than small base dies.

[Tzed250]

Get a Hornady headspace comparator and measure a fired round from your rifle. Adjust a standard, not SB, FL sizing die so it moves the shoulder back .003". Check for reliable chambering. SB dies or FL dies adjusted to far down are a recipe for short brass life. With correctly sized brass you should see a split neck or loose primer pocket long before any other problems.

Tool:

http://m.hornady.com/store/Headspace-Gauge-Kits/

[opsoff1]

Your symptoms point towards excessive HS, but do yourself a favor get and hard data before you start chasing this down the rabbit hole.
Excessive HS can be created by over sizing the brass (pushing the shoulder back to far) or it can be the result of the mechanical dimension issues; i.e. chamber cut too deep, bolt face too deep or improper dimensioned locking lugs.
First and foremost - find the headspace of your chamber. Minimum chamber headspace should be NO less than 1.464 and not exceed 1.467. 1.467 is the NO GO measurement. If you have a 556 NATO chamber, it shares the same minimum dimension, but can get as big as 1.470. (Don't go that large - no need). Pay no attention to Field or Field II dimensions - those are outside of what is acceptable for your use.
Once you know your chamber HS, then measure your cartridge headspace.
As you can see from above, you have basically a .003" window of acceptable CHAMBER headspace.
Your brass should ALWAYS be SMALLER (HS wise) than the chamber HS.
A good rule of thumb is the size the brass approximately .002 - .003" under chamber headspace. This will ensure two things, proper functioning and longevity for your brass.
If you sized your brass too much - you could very well have created an excessive HS problem. (As has been stated previously - ditch the SB dies - no need at all.)
After you have the chamber HS, find your cartridge HS; measure the brass BEFORE sizing it. Then measure your brass AFTER you size it.
If you are sizing it too much - readjust the sizing die to achieve the .002 - .003" under chamber HS dimension.
Measuring cartridge HS of a fired case is not accurate. Brass stretches during extraction from gas guns and the measurements generally show excessive numbers.

Once you have the hard data - compare the numbers.
To summarize:
Chamber HS: 1.464 - 1.467
Cartridge HS: .002 - .003 under the above dimension

Regardless of what your chamber is - if you cartridge is .010 - .020, .030 UNDER the chamber dimension - that is a guaranteed case head separation.

PM me if you need more technical info and assistance in measuring your chamber & brass.