Tight chamber leads to Kaboom!

[C4IGrant]

http://www.ar15.com/forums/topic.htm...=393112&page=1

Anyone want to bet that the DPMS had a tight chamber and there was a pressure spike??



C4

[MX5]

The half dozen DPMS bbls that I've run a finish reamer into were all marked '5.56', but there sure is a lot of metal coming out of them.

[jmart]

I'm not buying it Grant. That's 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.

[Robb Jensen]

Probably somehow fired slightly out of battery.

[jmart]

How can an AR fire OOB? I'm 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.

[Robb Jensen]

How can an AR fire OOB? I'm 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.

[Iraqgunz]

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 isn't their fault (my opinion only). It could also be a combination of tight chamber, ammo, etc...Voodoo?

[jmart]

If you read the whole thread, Hornady is being very stand up about it. They've inspected the other unfired cases from the OP's 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 didn't burst so I doubt there was a Silver Bear squib round stuck in the bore, but something is amiss. And I still think it's something much more than a tight chamber. Could be a combination of things actually.

[Robb Jensen]

I don't think it was a squib, as most that I've seen are much more catastrophic. It sounds like he really only perm. damaged the mag and the gun is 'locked up' 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 'Canned Heat'). I say slightly out of battery since the casing can't expand bigger than it's own chamber. I'm sure there's 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 = 'too tight', 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.

[Iraqgunz]

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.

I don't think it was a squib, as most that I've seen are much more catastrophic. It sounds like he really only perm. damaged the mag and the gun is 'locked up' 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 'Canned Heat'). I say slightly out of battery since the casing can't expand bigger than it's own chamber. I'm sure there's 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 = 'too tight', 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.