http://www.ar15.com/forums/topic.html?b=3&f=118&t=393112&page=1
Anyone want to bet that the DPMS had a tight chamber and there was a pressure spike?? 
C4
The half dozen DPMS bbls that Iāve run a finish reamer into were all marked ā5.56ā, but there sure is a lot of metal coming out of them.
Iām not buying it Grant. Thatās an ammo problem, either bullet setback or something seriously amiss, with either the Hornady cartridge or a squib from the preceding Silver Bear.
A brand new cartridge case can contain a decent pressure spike, but when something goes kaboom, pressures are well beyond what you would see from NATO ammo fired in a SAAMI chamber.
Probably somehow fired slightly out of battery.
How can an AR fire OOB? Iām trying to picture how the firing pin can protrude far enough beyond the bolt face to strike the primer, but the bolt not rotating and seating behind the locking lugs.
Broken firing pin or too much firing pin protrusion which is why there is a MINimum and a MAXimum on the firing pin protrusion gauge.
We had the same thing happen here just before I arrived in October 2007 with a Bushmaster M4gery. Unfortunately, no one documented the ammo or conditions. It blew the barrel extension apart and messed up both the lower and upper receiver. My guess is that there was possibly a squib round, although I would venture to say that it would have also caused a malfunction and it should have been noticed. Hard to say. Personally I would not have let anyone dick with it until I had taken photos and documented everything first.
Since he was firing different types of ammo and unless they can recover the casing (headstamp) Hornady could easily say that the damage isnāt their fault (my opinion only). It could also be a combination of tight chamber, ammo, etcā¦Voodoo?
If you read the whole thread, Hornady is being very stand up about it. Theyāve inspected the other unfired cases from the OPās lot and they determined the brass was good (no annealing issues). They also fired the remaining cartridges w/o issue. Next up is an inspection of the gun. Apparently the barrel didnāt burst so I doubt there was a Silver Bear squib round stuck in the bore, but something is amiss. And I still think itās something much more than a tight chamber. Could be a combination of things actually.
I donāt think it was a squib, as most that Iāve seen are much more catastrophic. It sounds like he really only perm. damaged the mag and the gun is ālocked upā which means the casing is expanded too much. How does this happen, firing slightly out of batter or too long of a round (as seen with āCanned Heatā). I say slightly out of battery since the casing canāt expand bigger than itās own chamber. Iām sure thereās other damage like a cracked bolt, damaged extractor, damaged CAM pin and/or firing pin. Possibly the carrier is cracked or carrier key and charging handle also may be damaged.
Factors that I believe contributed to this problem:
DPMS = ātoo tightā, because we (DPMS) think customers care more about accuracy vs. reliability, 5.56mm NATO is the way.
Steel cases = leaves a lot of stuff in the chamber (on DI guns) which might have been enough to keep the bolt from completely locking but just enough for the firing pin to light off the primer.
Robb,
I am just pissing in the wind on this. Would definitely like to see some pics and find out what damage was sustained and the end cause. Hopefully we will get some more poop when everyone is done doing the fingerbang.
Just posted this on TOS on a similar thread with the same ammo (Hornady purched through Cabellaās), where the OP had several primers pop in his RRA. The following pertains more to that type of situation than the more severe incident thatās being discussed here, but I still think itās somewhat relevant to the general issue of pressure spikes in general:
Could be a couple of things, bad ammo, tight chamber thatās not really a 5.56. Another thing it could be is a carbon ring thatās formed just in front of the throat/leade area.
Give your barrel a good cleaning. Use a good carbon cutting cleaner and/or some IOSSO bore paste or JB. Short stroke through the throat area with a tight patch and make sure it is absolutely spotless and cleaned down to the bare metal/chrome. Alternate with a bronze brush and scrub the area. Donāt assume the area is clean unless youāve been pretty aggressive previously ā some solvents just end up cleaning around the ring, but the ring remains. Get that area clean.
Some powders are worse than others in this regard. Even if you are one the guys who doesnāt clean that often and subscribes to the ājust keep lubing itā theory, it pays to ensure your locking lug area,chamber, and the first couple of inches of the bore are kept clean. JMHO.
Happens all the time. As the rep at Hornady said, they shot the rest of the ammo and there was zero issues. .223 chambers with 5.56 Pressure equals bad things. We see it all the time.
C4
Agree. NO squib, tight chamber and extra residue in the chamber from steel ammo.
C4
Popping primers is generally always because of tight chambers.
I have reamed just about every tier 3 AR barrel and I am here to tell you that none of the are true 556 NATO chambers.
C4
I donāt think you are dude,
I missed the part about Hornady getting it apart and finding no damage.
Iād like to see the parts and the rifle as well. Iām sure there more damage then Hornady is telling him. ARs donāt just vent gas through bottom of the chamber for no reason, I know he said the case ruptured but that shouldnāt really do anything except get stuck in the chamber. The bolt shouldnāt have unlocked before the pressures within the barrel had significantly dropped. I think itās probably a combination of things. Too tight and a very dirty chamber, a bad case, too big of a gas port, too light of a buffer (bolt moving too early)ā¦all for the perfect storm, if you will.
Papabri:
UPDATEBoth the AR and the remaining 298 rounds were sent to Hornady in Nebraska. Todd at Hornady has been very helpful in letting me know what they are doing.
He was able to get the spent case out, and it suffered a case separation at the rear of the case (sorry, no pics). What they donāt know as yet is what caused the separation.
Hornady did a lab test on the remainder of what I sent back and found no annealing problems. Todd shot the remainder of the box I opened and all fired without a problem.
He said that DPMS will get the gun from him and do their own inspection. Todd said that from what he could tell, there was no damage to the rifle. They were also going to do some sort of chamber cast to see if there are any deficiencies with the chamber.
Iāll provide another update when I know what the next step is. Iāll ask Todd at Hornady about the suspicion about soft points. Iām no expert, but it would seem like a soft point, as opposed to a FMJ, would take up the pressure if it misfed, and not push the bullet further into the case.
Itās pretty damn weird to me. Iāve only seen guns this has happened to and it was all āCanned Heatā ammo which uses military cases that are often from M249s which lengthens them and makes them weaker.
What happened here does not happen all the time. This was a minor kaboom. Minor kabooms happen for reasons other than just a tight chamber.
Iāll grant you that primers can pop, caseheads after multiple reloadings can separate (although virgin brass shouldnāt), etc., but you donāt have minor explosions going on in those instances. Thereās something else at work here. (The 2nd thread BTW had some interesting happenings, same ammo, same lot).
It could have been a bad round (light charge weight) that resulted in SEE. The rest of the ammo could have been fine. It could have been a minor bore obstruction. It could have several things that led to āthe perfect stormā as Robb mentioned, but a SAAMI chamber by itself doesnāt cause a kaboom.
It couldnāt possibly have been an out of spec chamber because it was a DPMS chamber and they have 5.56 Nato chambers, and supply parts to the .mil, and are just as good as the Colts and LMTās and Noveskeās you sell, and are alot cheaper. 
Seriously though, maybe he can pick up the tab for his friends Dr bills for his finger, with the money he saved buying the DPMS.
-RD62
Iāve been following this on TOS and I think it sounds like a tight chamber. The guns it happened in are somewhat known for their non-5.56 chambers (RRA and DPMS). I think the gun at Hornady was going to have a cast of the chamber taken - that will be quite telling.
I saw this ammo on the shelves at Cabelaās in DFW the other day. 50 rounds to the box and looks just like the practice ammo (box only). I know Hornady makes you sign a waiver to get the real 5.56 TAP and I wonder what this stuff really is. On TOS someone mentioned it was an overrun from a contract with an LE agency in Kentuckyā¦
Spooky
Geez, tight chambers, large ports, unstaked castlenuts you guys are driving
me crazy!:eek:
Okay,Okay, I give up and give in. Iāll take my RRA entry tactical as backup for
the CDM4LE this week to carbine class. After that its history:( . Iāll
forget about the fact that I shot several sub-moa groups with it the first time
out to the range with it. Also that I love that RRA 2-stage trigger 
.
After reading this and other threads I going to lose sleep worrying about
switching between the decent quantity of AE 55gr and XM193 I have on
hand.
Have to do something, Grant put me on your list for a BCM middy! E-mail
coming! Iām starting to feel better already:D .
Amen.