I've been reloading thousands of 5.56mm LC 09 brass for my SIG 556 NATO rifles with 16" 1:7" twist barrels (using 25.0 grains of Hodgdon BLC-2, 75 grain Hornady BTHP, and Wolf SR primers) using my RBCS full length .223 die and normally getting 5-6 reloads before the case necks split and I toss the case. Accuracy, performance, and reliability has been outstanding.

However, I purchased a custom 1:7" twist 20"-.223 Match Barrel for my Thompson Center Contender rifle and tried shooting these same reloads in this custom .223 barrel and this is what happened on the 2nd reload (new LC '09 brass loaded and shot in the 556's and then reloaded and shot once in the Thompson Center):





After much discovery using my RCBS Precision Micrometer and Dillon Headspace gauge, I found that cause of this problem was that I was full length resizing the cases for the 556, pushing a case expanded out to +0.003-0.005" back down to 0.000.

Then when I fired them in the SIGs5.56mm  NATO chamber, they worked just fine, no problems for 5-6 reloads when the case necks would start to split.

However when I then fired these reloads in the .223 Thompson Center rifle, they expanded out to 0.008-0.010, which is TOO MUCH.

Now I just resize the cases from the T/C .223 back down to 0.006 and they work fine in this rifle, but not in the SIGs.....


Apparently the SIGs all have a tight chamber and the T/C's .223 chamber is on the long side, unless this is due to some inherent difference in the NATO vs. .223 chamber size?

I ended up getting a Dillion carbide .223 full length resizing die and setting it up to use on the longer T/C chamber specifically for the T/C cartridges only.

Now I keep the cartridges segregated.

I still use the same recipe for all the different rifles and no signs of pressure or anything else dysfunctional occurs.

End of problem....

You have just clearly explained why you do not fire a round sized for one rifle in another rifle.  Just another basic of handloading.

One more quick question, are these incipient case head separations that far up the case very dangerous?

I would assume that any case head separation would be dangerous, but in this case it did not appear that any gas was ejected back from the case split?

Just wondering as this won't be happening to me again anytime soon....

There are head space shim kits available for the TC from Bellm.

Case head separation in a crack barreled rifle could be very serious.  The gases will be escaping through that gap right next to your face.  

If you look at the MilSpec specified brass hardness profile for the M855 NATO round, the point for maximum hardness and minimum thickness of the case body occurs just about where the four on the left are marked in your photos.  I am sure this is not mere chance coincidence.  Your photos are useful in that they show that the impending separation point is not down toward the case head, where many assume it is going to occur.

"Case head separation" as you demonstrate it, normally, is not catastrophic, because the failure occurs at or near chamber pressure peak and the brass is still sealed radially against the chamber walls even though it is in two pieces at that point.  After the event, you see no soot or hot gas trails eminating from that point outside the case.

"Case head failure" is the serious failure and is often catastrophic.  The rear bulkhead cracks open at or near the "web" and the primer pocket.  This occurs at a point that is at, or near, the unsupported end of the case (approx.0.00 to 0.20" out of the back of the chamber for both autoloaders and standard bolt action designs.)   There is no chance to maintain seal within the chamber at that point.   High pressure plasma vents out the back and into the action with extreme prejudice.  Fortunately, these failures are rare, and may be caused by a structural fault of the brass, and/or extreme overpressure.

Thanks for the outstanding information on case head separation and failure, I feel much better about it now that I have it resolved.

Here is some more Thompson Center Contender PORN to enjoy!!



One Hornady 75 grain BTHP hand load @ 125 yards....

The T/C is a virtual laser.

Where are these case measurements being taken?
How is the powder being measured?
Where are the chrono readings for both rifles?

I remember your new TC - cool rifle.  Wish it was more compatible with your BL-C2 loads.   It would be interesting to see how both rifles handle a 5.56 Go & No-Go gauge...  

I see on Hodgdon's load site that a max Varget load gives a 50fps higher velocity reading at 1,100 psi LESS pressure than BL-C2 –– might want to switch-to it.

I`ve had more than one of those case head separations happen after loading 5.56 Mil spec brass more than 5 times for an AR. After 5 reloads, the brass goes in the recycling bucket.

Some folks have loaded theirs more times than what I get out of my reloads. Still trying to figure that one out.

The RCBC Precision Mic readings were done on the fired cases from both the SIG 556 NATO and the T/C .223, it was readily apparent that the SIG 556 NATO chamber was a tight chamber compared to the T/C's....I also measure the powder individually to the 25.0 grains, not 24.9 or 25.1 grains - just 25.0 for each one of the thousands I've got.

Here is a nice review of how to use the RCBS Precision Mic to determine headspace in your chambers:

http://www.realguns.com/archives/035.htm

I don't have any chrono data for my reloads but have never had any problem with them over the years.

All the SIG 556s and the Thompson Center have a 1:7" twist barrel for the heavier 75-77 grain OTMs, which is all I shoot in these rifles.

I have talked to a couple of Thompson Center people and they specifically told me -> DO NOT FIRE 5.56mm NATO ammo in your T/C Match .223 barrel, apparently the locking mechanism locks up and you cannot break the action without taking it to a gunsmith. So, I don't need to try that to find out...

I've never encountered incipient case head separation before in thousands of these same reloaded rounds for my SIG 556 NATO's, only when I resized a case shot in a NATO chamber and then shot it in my T/C .223 chamber have I had this occur.

Skypup,

I don't understand what ".223 Match Chamber" means for your T/C, especially given your measurements of the case-stretch i.e. massive shoulder datum-point movement of .008-.010 on cases coming out of the T/C.  I believe the T/C has a chambering problem.

I still suggest getting a 5.56 No-Go gauge, and even a 223 No-Go gauge.  My guess is neither will prevent the T/C action from closing, meaning there is a serious problem.

Here's some info I saved from http://www.bushmaster.com/faqs/afmviewfaq.aspx?faqid=728
For proper headspace the bolt should close on the "GO" gauge and not close on the "NO-GO" gauge. If the bolt closes on the "NO-GO" gauge it indicates that there may be chamber and/or bolt wear and there may be a degrading of accuracy. If a bolt closes on a "FIELD" gauge it would indicate a dangerous condition where cartridge cases will split or separate.
...  
Our chrome lined barrels are chambered to 5.56mm NATO Spec. NATO Headspace gauges are used. The NATO "GO" gauge is 1.464", "NO-GO" is 1.470" and "FIELD" is 1.474".   SAAMI .223 Headspace gauges are different. SAAMI "GO" gauge is 1.464", "NO-GO" is 1.467" and "FIELD" gauge is 1.470". If SAAMI gauges are used the "FIELD" gauge is the same as the NATO Spec "NO-GO" gauge and can be used as such.

Notice that your measurements match the NATO FIELD gauge spec (and exceed the SAAMI 223 FIELD spec by 66%) for a dangerous-conditioned barrel that should be withdrawn from use.

I hope you see/follow up on this, so you and the (rechambered & corrected) T/C continue to go on many hog hunts together.

Chris65, thanks for your reply, I will investigate the T/C's .223 chamber with some gauges from Brownell's.

Update?

They may have measured their shoulder length and are being careful to minimally move the shoulder back instead of moving it back to fir any chamber.

You only need to move the shoulder enough to get reliable chambering.

In a bolt gun this might be zero, in a semi you usually need at least a  thousandths and sometime a few thousandths (for a really rough chamber maybe even more).