Pmag explodes

[NapalmCheese]

I'm not an AR guy, but that bolt face in the first pic looks like it has taken a beating with maybe incomplete chambering or off center seating of rounds? The mag was probably an innocent bystander.

It looks like the case head separated and is stuck in the bolt. I'll revise my guess to OOB or headspace issues.

 

[jarhead714]

Be thankful. It blew up as intended; out the bottom of the floor plate. Reason #1 to never hold onto the magazine when shooting.

 

[Deleted member 16735]

Out of battery detonation maybe, what did the case look like from that? It's not even possible to double fill a 556 case it would have to be the wrong powder completely. I've seen people report AAC ammo exploding rifles though.

Edit: oh that's the end of the case still on the bolt, yeah obb most likely. Could be a trigger issue

 

[Walmart_AR15]

Yup, OBD. The case separated at the base because it is no longer supported by the chamber, releasing gas back into the receiver. The receiver and mag worked as designed and vented the gas downward instead of splitting the receiver and cause serious injury to the shooter.

This can happen when the bolt bounce. If it were a parts gun, then the buffer may not have the proper internal weights (ex. using a 9mm buffer). The dimension of the lower to the upper may also be off. Typically, when AR bolt is out of battery the hammer's angle is off so it cannot strike the firing pin.

Another possibility for OBD is stuck firing pin. The difference between the two is one blew up when you pull the trigger, the stuck FP will blow up after the shots fired and then it blew up.

I'll be surprised if the gun were a reputable manufacturer.

 

[Shredla]

@Grizzly308
im saying this In the most respectful way possible-
By withholding the name of the maker of both the "factory built upper" as well as the ammunition you were using, you're only protecting the company. Maybe it was a one day "off" quality control issue, There could be multiple firearms that are out of spec (if Thats the issue), and it does happen from time to time.
You're not going to hurt sales if you're worried about protecting the company, but "maybe" someone might know to check their Headspace/trigger/firingpin/BCG/ammo etc if they have the same setup.

 

[JustADiversityHire]

Don't understand why you would withold the manufacturers in this case.. Everyone listed the possible reasons but the manufacturers could very well be the problem. Even "good" manufacturers will have bad batches of stuff here and there. Sharing the manufacturers for the ammo and upper could help identify faulty lots of ammo or parts. This is how recalls are made and it's a disservice to fellow consumers to not report issues.

 

[Deleted member 16735]

Yup, OBD. The case separated at the base because it is no longer supported by the chamber, releasing gas back into the receiver. The receiver and mag worked as designed and vented the gas downward instead of splitting the receiver and cause serious injury to the shooter.

This can happen when the bolt bounce. If it were a parts gun, then the buffer may not have the proper internal weights (ex. using a 9mm buffer). The dimension of the lower to the upper may also be off. Typically, when AR bolt is out of battery the hammer's angle is off so it cannot strike the firing pin.

Another possibility for OBD is stuck firing pin. The difference between the two is one blew up when you pull the trigger, the stuck FP will blow up after the shots fired and then it blew up.

I'll be surprised if the gun were a reputable manufacturer.

Incidentally bolt bounce is common when the spring is too heavy and the bolt goes into battery too fast (and out again when it bounces). I only bring this up for others reading that think putting a heavy spring will help slow down the bolt. It will on extraction but not on return because it has more stored energy so it actually speeds up. Heavier buffer is generally what needs to be added to slow down bolt speed in both directions, or my preference which is adjustable gas block.

 

[AR15barrels]

I was shooting my AR15 rifle using factory 556 ammo. Rifle had about 100 rounds in the year Ive had it and have always used a standard Pmag.

After about 20 rounds (shooting 5 rounds in between) of shooting without any issues, the magazine exploded. I discovered the BCG was cracked and had pieces broken off.

Any thoughts of what caused this?

The Mag did not explode.
The cartridge in the chamber exploded.
When the bolt carrier peeled open, the pressure then vented down into the magazine and that's what made the mag self-disassemble.

What you had is called a high pressure event.
It could simply be one bad cartridge or it could be that you had a squib round which left a bullet in the bore and then you loaded another live round behind it and fired it.

 

[AR15barrels]

It looks like the case head separated and is stuck in the bolt. I'll revise my guess to OOB or headspace issues.

That's not an OOB.
OOB are much lower pressure since the case is not all the way into the chamber and the case rips apart well ahead of the case head like this:

His photos show a case failue from an in-battery overcharged case or bore obstruction.
When you get a case head tearing open at the unsupported extractor cut in the bolt, that's always an in-battery high pressure event.

The way a case head gets snapped under the extractor on an AR is for the case to be stopped in the chamber so the bolt carrier can push the bolt forward to get the extractor to snap over the case rim.
The ejector won't let the case get snapped under the extractor without significant force so the firing pin can't get to the primer until the case rim has snapped under the extractor.
That's why it's almost impossible to get an OOB in a locked breech AR.

The case HAS to be in the boltface for the bolt to be supporting the case head and have the unsupported extractor area blow out.
In order for the pressure to get high enough to cause the case head to rip off the case body and the ejector area peel open, the case has to be locked in the chamber to hold the pressure it takes to get the rear section of the case to fail.

You never ever get that type of case failure in an OOB because the case being IN the chamber is a requirement for that type of case failure.

 

[L4Dtwo]

Define "Well known ammo"

There's a lot of "Well known" sellers that sell remanufactured ammo.

 

[Shredla]

Well known gun show reloader maybe?
How well known are we talking..
Bubba's ammo strikes again.

 

[ar15robert]

A mag just doesn’t blow up. I’m not a pmag fan either. My bet is an overcharged round blew the case and that caused the mag to break apart.

No matter how good the ammo maker is a mistake can happen and that’s most likely the cause

 

[Walmart_AR15]

Many years ago, I had a "high pressure" event. Not only did the mag self-disassembled, upper receiver was cracked along top of the carry handle and peeled my bolt carrier bottom open, exposing the bolt, into the mag well. Bulged the mag well for good measure as well. The bolt actually looked fine except the extractor was gone. The cause was OOB ignition, and it was jammed open by the next round in the mag (how? but was amazingly still jammed between the BC and the receiver when I try to separate the parts). Barrel looks perfectly normal. Found the blown-out case head on the ground after I pull out the bolt. My ears were ringing for two days.

I recycle the barrel as a dedicated 22 just in case. Lower still function as RAW.

It was back in the days many manufacturers just started to jump into the AR business. Tolerances between companies are less than desired even tho everyone advertised mil-spec.

 

[Walmart_AR15]

Heavier buffer is generally what needs to be added to slow down bolt speed in both directions, or my preference which is adjustable gas block.

The key is the weights in the buffer. Those loose metal moving inside the buffer. Those weights counter act against the bolt bounce and close the bolt for you.

As the BCG and buffer move forward, the weights inside the buffer lags behind a bit. When the bolt bounce, the weight is now caught up and "hammer" shut the bolt again. Thus, it is important to make sure those weights exist inside the buffer.

It is the reason why it's a bad idea to use a heavy 9mm bolt instead the proper carbine bolt with internal weights to save money.

 

[AR15barrels]

Many years ago, I had a "high pressure" event. Not only did the mag self-disassembled, upper receiver was cracked along top of the carry handle and peeled my bolt carrier bottom open, exposing the bolt, into the mag well. Bulged the mag well for good measure as well. The bolt actually looked fine except the extractor was gone. The cause was OOB ignition, and it was jammed open by the next round in the mag (how? but was amazingly still jammed between the BC and the receiver when I try to separate the parts). Barrel looks perfectly normal. Found the blown-out case head on the ground after I pull out the bolt. My ears were ringing for two days.

I believe that your ascertaining of an OOB as the cause is incorrect.
You can not reach the level of pressure required to do the described damage without the bolt being locked closed on the chamber so that the chamber can contain the pressure as it builds high enough to do all the damage.
When the bolt is NOT locked closed, the case vents the pressure well ahead of the case head and you don't get carrier or receiver splitting or lower receiver magwell bulging.

 

[ef9boy88]

Gun exploded PMAG took the hit. Better than the other direction.

 

[han_solo]

How many times was that case reloaded ?

 

[DesertDepotArms]

Junk ammo, magtec?

 

[2muchsoysauce4allthisrice]

still havnt listed manufacturer of upper and also ammo?

 

[jarhead714]

still havnt listed manufacturer of upper and also ammo?

Further shaming is needed. Might as well have not even started the thread.