6.5 upper build went kaboom

[matty86suk]

Edit..... UPDATE, turns out I unknowingly had an armalite pattern BCG installed in a DPMS pattern receiver.....


Today my 6.5 upper build had a kaboom, details
I pieced this together, this wasn’t a factory gun
Stag 10 upper receiver, dpms pattern
Stag 10 22” 6.5 barrel, rifle length gas system
Brownells 308 BCG, headspace was checked
Stag 10 lower with rifle buffer and spring, luthar stock

The kaboom happened on exactly the 75th round fired, the rifle ran flawless until the kaboom and was grouping under 1 MOA with factory ammo when I did my part. I was shooting federal 140gr non typical ammo when the kaboom occurred.

Pressure vented down the mag well and destroyed a 10 round Pmag, I had my hand on the mag and my left pinky finger has a small cut and feels like it was hit with a hammer but no other injuries, thank god.

I was shooting 5 round groups and this was the last round fired for the grouping, it did not slamfire, I pulled the trigger, I have no idea what went wrong??

 

[matty86suk]

Primers have some cratering, and slightly flattened but no other real issues with the spent brass.

 

[km77]

Dang man. Glad you’re ok, definitely could have been worse!

 

[Constructor]

The rifle didn't fail the case ruptured.

 

[Steel head]

Yikes!

 

[Snuby642]

Good case for proper eyepro .

Catastrophic failure from factory round. Wow!
Hope you get to the bottem of cause.

Would like to see mfg response on this.
Quarantine remaining ammo and box.

Glad you are ok.

 

[TACC]

This was with factory ammo you stated, not reloads correct?

 

[RTH1800]

It’ll buff right out! ?

 

[lte82]

The bolt lugs we’re not lined up with the barrel extension when the round went off. Bolts tend to crack around the cam pin, which can cause this, though yours looks ok. A barrel extension can lose the index pin and rotate a bit, also causing it. I’ve also seen an upper with a mis-timed barrel extension index pin slot cause it. IE it should be at 12:00 and it was cut at like 12:15. Less likely, the bolt carrier failed which allowed the bolt carrier to go all the way forward but not force the bolt to rotate all the way.

 

[TheGerman]

Hot loads.

 

[Constructor]

The bolt lugs we’re not lined up with the barrel extension when the round went off. Bolts tend to crack around the cam pin, which can cause this, though yours looks ok. A barrel extension can lose the index pin and rotate a bit, also causing it. I’ve also seen an upper with a mis-timed barrel extension index pin slot cause it. IE it should be at 12:00 and it was cut at like 12:15. Less likely, the bolt carrier failed which allowed the bolt carrier to go all the way forward but not force the bolt to rotate all the way.

The carrier can not go all the way forward without turning the bolt 22.5 degrees.
The firing pin cannot reach the primer unless the bolt is all the way in the carrier pushed to the rear and has been turned 22.5 degrees.

 

[lte82]

The carrier can not go all the way forward without turning the bolt 22.5 degrees.

If the bolt breaks at the cam pin, the bolt head may not rotate.

The barrel extension may not be indexed properly as well. Either via the index pin breaking or the relief on the upper receiver not be timed exactly at 12 o’clock.

 

[Constructor]

If the bolt breaks at the cam pin, the bolt head may not rotate.

The barrel extension may not be indexed properly as well. Either via the index pin breaking or the relief on the upper receiver not be timed exactly at 12 o’clock.

The bolt isn't broken, not at the cam pin not the lugs. The barrel extension isn't broken. The barrel didn't explode. The only thing that failed was the case and when it released the pressure it broke the carrier which broke the upper receiver nd blew out the mag.

 

[Constructor]

It may be possible the barrel was ported too large creating an over-gas issue so bad the carrier started moving to the rear while pressure was still high in the chamber. If it moved to the rear far enough to unlock the bolt and the bolt pulled the case to the rear exposing too much of the case, the case could not handle the pressure.

 

[hypno02]

Tell federal you expect them to buy you a MWS 6.5. Looks like an overcharged load to me but at the same time I’m surprised the damage is that extensive. Glad you’re ok!

Buying all factory rifles from now on??????

 

[aslrookie]

I doubt Federal will take any responsibility for it. Especially since it was a gun you put together.

 

[flyright]

Brass in the extension looks like the bolt was unlocked at the time of the kb. Brass was definitely not supported by the chamber but I guess that's obvious. The Fed stuff is about as hot as H superformance so nothing new there. I doubt you'll get anywhere with a manufacturer. Glad you're okay, man.

 

[Anthonyc]

Wow, glad you’re ok!

 

[10ring1]

Glad you are ok! That sucks. Federal will probably reimburse you in some way. Hope that isn't blood on that cardboard! Is it? Did u get hurt?

 

[5pins]

Was the headspace checked after it was assembled?

 

[sandwarrior]

That's strange. The carrier is busted open, but almost no damage to the bolt. Did the retaining/pivot pin for the extractor break? Or, did you just pull it out and not picture it?
I know you were shooting groups, but did you see the round previous hit the target? Did it seem like a soft round? Reason I ask is, was there a possibility the previous bullet didn't go out the barrel? Was every shot the same recoil and you counted holes?

The only thing I can think of off the top of my head is too much gas coming back too fast. I bet if you dye-penned the bolt you'd see some cracking around the cam pin. Or, you just had a weak carrier.

Anyhow, glad you got out of it with minimal injuries. A lesson to all of us to keep that eye-pro on even when you think it hinders.

 

[lte82]

The bolt isn't broken, not at the cam pin not the lugs. The barrel extension isn't broken. The barrel didn't explode. The only thing that failed was the case and when it released the pressure it broke the carrier which broke the upper receiver nd blew out the mag.

100% agree, the bolt and barrel ext index pin are in tact. But the fact that the bolt didn’t shear the lugs (which is what happens if it were over pressure) and the carrier is destroyed means the lugs were not lined up when it went off. The bolt thrust killed the carrier because the bolt was not properly timed to the extension.

OP check to see if the slot where the barrel extension index pin slides in is exactly 12 o clock on the upper receiver.

 

[sandwarrior]

100% agree, the bolt and barrel ext index pin are in tact. But the fact that the bolt didn’t shear the lugs (which is what happens if it were over pressure) and the carrier is destroyed means the lugs were not lined up when it went off. The bolt thrust killed the carrier because the bolt was not properly timed to the extension.

OP check to see if the slot where the barrel extension index pin slides in is exactly 12 o clock on the upper receiver.

That's a good point.

OP
I know it's only 75 rounds through the gun, but is there a possibility of getting close-up pics of the bolt face and rear of the bolt lugs?

 

[Jackomason]

So... how was the last group?

 

[supercorndogs]

Anyone taking a close look at the broken off piece of the carrier? Looks like a possible defect in the casting on one corner opposite the oil holes.

 

[whatsupdoc]

My take on this is the bolt was not fully rotated (locked) the rifle fired, bolt driven back into bolt carrier at high speed breaking the bolt carrier at the cam pin cut. Case head blows. Carnage.

It is not supposed to happen by design but if there is enough tolerance stack up it maybe possible.

 

[matty86suk]

That's a good point.

OP
I know it's only 75 rounds through the gun, but is there a possibility of getting close-up pics of the bolt face and rear of the bolt lugs?

 

[matty86suk]

So... how was the last group?

The last 4 round group, the fifth round kaboomed.

 

[matty86suk]

100% agree, the bolt and barrel ext index pin are in tact. But the fact that the bolt didn’t shear the lugs (which is what happens if it were over pressure) and the carrier is destroyed means the lugs were not lined up when it went off. The bolt thrust killed the carrier because the bolt was not properly timed to the extension.

OP check to see if the slot where the barrel extension index pin slides in is exactly 12 o clock on the upper receiver.

Everything appeared aligned during the build, feed ramps lined up perfectly, barrel but was torqued to spec using the proper tools, headspace was checked.

 

[__JR__]

just out of curiosity, check for wear on the hammer and if the firing pin protrusion allows you to ever get the fp thru before hammer strike. Doesn't look like the cartridge itself can be examined, and super glad you still have all your fingers and face!!!!

 

[matty86suk]

My take on this is the bolt was not fully rotated (locked) the rifle fired, bolt driven back into bolt carrier at high speed breaking the bolt carrier at the cam pin cut. Case head blows. Carnage.

It is not supposed to happen by design but if there is enough tolerance stack up it maybe possible.

The primer impact on the failure is really deep vs. a non failure.

 

[Blutroop]

It may be possible the barrel was ported too large creating an over-gas issue so bad the carrier started moving to the rear while pressure was still high in the chamber. If it moved to the rear far enough to unlock the bolt and the bolt pulled the case to the rear exposing too much of the case, the case could not handle the pressure.

This^ I agree. Look at the brass blown into the chamber. This is the best analysis I’ve seen of what happened.

 

[matty86suk]

just out of curiosity, check for wear on the hammer and if the firing pin protrusion allows you to ever get the fp thru before hammer strike. Doesn't look like the cartridge itself can be examined, and super glad you still have all your fingers and face!!!!

No usual wear on the trigger parts, he buffer takedown pin/retainer is bent though

 

[matty86suk]

That's strange. The carrier is busted open, but almost no damage to the bolt. Did the retaining/pivot pin for the extractor break? Or, did you just pull it out and not picture it?
I know you were shooting groups, but did you see the round previous hit the target? Did it seem like a soft round? Reason I ask is, was there a possibility the previous bullet didn't go out the barrel? Was every shot the same recoil and you counted holes?

The only thing I can think of off the top of my head is too much gas coming back too fast. I bet if you dye-penned the bolt you'd see some cracking around the cam pin. Or, you just had a weak carrier.

Anyhow, glad you got out of it with minimal injuries. A lesson to all of us to keep that eye-pro on even when you think it hinders.

Yes, extractor pin failed or came out, I didn’t find it. 4 rounds on paper, fifth kaboomed. I’m thinking I am over gassed but I had no real indications of being over gassed. Spent brass in good shape, ejection was at 6 o’clock 2-3ft away consistently. I was wearing eye pro, but they are not ballistic lens.

 

[thatguyouknow]

Today my 6.5 upper build had a kaboom, details
I pieced this together, this wasn’t a factory gun
Stag 10 upper receiver, dpms pattern
Stag 10 22” 6.5 barrel, rifle length gas system
Brownells 308 BCG, headspace was checked
Stag 10 lower with rifle buffer and spring, luthar stock

The kaboom happened on exactly the 75th round fired, the rifle ran flawless until the kaboom and was grouping under 1 MOA with factory ammo when I did my part. I was shooting federal 140gr non typical ammo when the kaboom occurred.

Pressure vented down the mag well and destroyed a 10 round Pmag, I had my hand on the mag and my left pinky finger has a small cut and feels like it was hit with a hammer but no other injuries, thank god.

I was shooting 5 round groups and this was the last round fired for the grouping, it did not slamfire, I pulled the trigger, I have no idea what went wrong??

I would assume blown out case/insanely hot load, but it seems like significantly more qualified people have shared their opinions already. Glad to hear you got out with only a few scratches you can probably salvage that lower and most of the parts.

 

[__JR__]

This^ I agree. Look at the brass blown into the chamber. This is the best analysis I’ve seen of what happened.

Yeah, curious on port diameter too

 

[Blutroop]

Yeah, curious on port diameter too

A lot of manufacturers over gas so it will be able to run all types of ammo and dirty chambers, ect. You’d be surprised how little gas you actually need though. I built a 17 Remington upper with a 26” barrel and a +3” rifle length gas tube and adjustable gas block and it’s impressive how light I can make the recoil.

 

[sandwarrior]

Yeah, curious on port diameter too

I think Ite82 and constructor mentioned that above. What always frustrates me is I dont have a math formula to go on. Justa lot of guess and by golly. I do know you can over gas while either over or under pressure in the chamber.

I shredded some Grendel brass that was under pressure due to cold and temp sensitive powder. It was high when it hit the bolt carrier! But, not enough to do the damage you’re seeing

 

[__JR__]

pressure curves vary

 

[Constructor]

To clarify. Take your BCG out of the upper. Pull the bolt forward as it would be when the carrier is to the rear. Holding the carrier push the firing pin forward. Does it protrude through the face of the bolt so it can strike the primer? So keep your finger on the firing pin pushing forward and slowly push the bolt back into the carrier . Here's the thing, that bolt CAN NOT TURN UNLESS it is between the breech and barrel extension lugs. How far did the bolt have to turn before the firing pin sticks out enough to hit the primer? Right, the straight part of the cam slot in the carrier is just long enough to keep the firing pin from hitting the primer until full 22.5 degree rotation has occurred.

 

[Constructor]

Port size on a rifle gas Creed should be .086-.089. A 223 should be .093 but a Creed has a lot more slower burning powder keeping the pressure high further down the barrel.

 

[unicron]

Yipes!!!! Glad you are ok. Thx for sharing

 

[bschneiderheinze]

Brass in the extension looks like the bolt was unlocked at the time of the kb. Brass was definitely not supported by the chamber but I guess that's obvious. The Fed stuff is about as hot as H superformance so nothing new there. I doubt you'll get anywhere with a manufacturer. Glad you're okay, man.

This On large frame ARs I will lube the shit out of them and cycle by hand until they go into battery freely (300-500). Clean them up lightly Lube and make sure it will go into battery easily. I would say your rifle ran dry were you running a can.

 

[just browsing]

My take on this is the bolt was not fully rotated (locked) the rifle fired, bolt driven back into bolt carrier at high speed breaking the bolt carrier at the cam pin cut. Case head blows. Carnage.

It is not supposed to happen by design but if there is enough tolerance stack up it maybe possible.

I am leaning this way as well, though a slightly different explanation. All of this is dependent on being wayyyy over-gassed though, and I’ve never seen an over-gas event cause that much carnage.

Bolt unlocks while pressure is still way high in the chamber. Case is still expanded, causing the extractor to either jump the rim or shear off due to retaining pin failure before the case can contract. Case head separates. Kaboom.

Aftermatch looks like classic case head separation, but whether or not it was caused by firing OOB is impossible to tell.

 

[Constructor]

Yeah I'm betting Hillary had something to do with it.

 

[bfoosh006]

Whose BCG is that ? ... and did you install an adj. gas key ?... I don't recognize the squared off back end of the gas key.

SunDevil ADIGS ?

Also that is an odd FP.

It looks like a Armalite FP, with out the spring around it.

I am asking because every Brownells 308 BCG I have seen has 2 gas exhaust ports.. not 3 in a row.

I am not saying you are wrong... but , do you have a link to the BCG you bought ?

The new Retro 308 BCG has 3 inline ports, and is DPMS pattern.. but I only see it in completely chrome coated.

 

[Soulezoo]

The last 4 round group, the fifth round kaboomed.

Did you call the flier?

 

[kenny1773]

Brownells 308 BCG, headspace was checked

Crap now I am starting to wonder if there is something up with the Brownells BCGs
These are supposedly made by Aero for Brownells

I have a thread here were I was having signs of over-pressure in my AR10 with a specific S&B 6.5 140gr ammo
Got lots of good advice here
Mailed ammo to S&B they said it shot fine in their guns and said it must be my gun

Hornady 140ELD factory ammo appears to work fine in mine SO FAR

but now I am wondering if the bolt may be contributing to the issues??

I checked and I do have a high pressure bolt in mine (checked by calling Brownells and confirming what I bought, and also my measuring the firing pin tip at .069-.070 which is the smaller 308 firing pin)

I think I may change my bolt to either JP or RCA (leave the carrier alone)

 

[bfoosh006]

Crap now I am starting to wonder if there is something up with the Brownells BCGs
These are supposedly made by Aero for Brownells

I have a thread here were I was having signs of over-pressure in my AR10 with a specific S&B 6.5 140gr ammo
Got lots of good advice here
Mailed ammo to S&B they said it shot fine in their guns and said it must be my gun

Hornady 140ELD factory ammo appears to work fine in mine SO FAR

but now I am wondering if the bolt may be contributing to the issues??

I checked and I do have a high pressure bolt in mine (checked by calling Brownells and confirming what I bought, and also my measuring the firing pin tip at .069-.070 which is the smaller 308 firing pin)

I think I may change my bolt to either JP or RCA (leave the carrier alone)

I truly do not think that is a Brownells 308 BCG. Just waiting to hear from the OP.

 

[matty86suk]

Whose BCG is that ? ... and did you install an adj. gas key ?... I don't recognize the squared off back end of the gas key.

SunDevil ADIGS ?
View attachment 7129149

Also that is an odd FP.

It looks like a Armalite FP, with out the spring around it.View attachment 7129140

I am asking because every Brownells 308 BCG I have seen has 2 gas exhaust ports.. not 3 in a row.

I am not saying you are wrong... but , do you have a link to the BCG you bought ?

The new Retro 308 BCG has 3 inline ports, and is DPMS pattern.. but I only see it in completely chrome coated.

You are right, I have(had) an armalite pattern (BCL) BCG in my DPMS pattern receiver. I bought the BCG from a gun show and it was displayed in brownells plastic packaging. It was slightly used when I bought it and the guy I bought it from said it saw less than 100 rounds in his “ar10”. I bought it thinking it was good to go in my dpms pattern AR10 and almost lost some fingers. I’ll admit I am a bit of a rookie putting ARs together but I am a mechanic by trade...

I have learned a costly and dangerous lesson about large frame ARs.