AR10 - Case head blowout

[awpk03s]

Hoping for a little help/advice diagnosing something that happened to me today while shooting.

Rifle - S&W MP10 .308 Win. Since my last outing the only changes to the rifle were a new scope (Vortex Strike Eagle 1-8x), and a JP SCS (1 tungsten, 2 Steel, medium spring). Other prior alterations are Geisselle trigger, and MI free float rail, and Geisselle gas block, and Area 419 Hellfire brake. The purpose today was to dial in the new scope, and evaluate the SCS.

Ammo - Atomic Ammo 168gr. Factory ammo that is in once fired Lake City brass and using Nosler Custom Comp bullets. I bought cans of 200 count from CMP several years ago.

I fired a couple of rounds normally, and then got one with big blast of gas and smoke and the magazine and guts (Larue) blew out the bottom of the gun. Case head was torn off, and crack on the shoulder of the case. I took down the upper and inspected the barrel and bolt and lubed them up again. I took a couple more rounds from the can and using a different magazine (Lancer), again got couple normal shots, and again big blow out through the magwell with the same case results.

This time I quit. Brought the rifle home and cleaned it, besides a lot of brown carbon buildup all inside everything, looks none the worse for wear.

So, please help me with some advice on the potential causes for this... ammo related seems likely to me, but I’ve been shooting this stuff off and on with no real prior problems. My only other change as I mention is the SCS. I don’t think that is the culprit here? Should I try lighter or heavier spring on it? Change the weights? I really don’t think that has anything to do with it though? But - again, please help me with some thoughts.

Thanks.

 

[W54/XM-388]

Most people would stop shooting that rifle / ammo after the FIRST kaboom and find out what the hell was going on..........

 

[awpk03s]

I do realize the stupidity in my persistence with it, and I should have pointed that out also.

 

[flyer]

I would do a web search to see if anyone else is blowing up that ammo.

Also, think about how it was stored. If it was subject to a lot of vibration that can change the burn rate of the powder.

The recoil system shouldn't cause a problem like this but did the blown cases extract and eject? Any extractor marks on the rims?

If you had a recoil system so light that you got extraction while the chamber pressure is still high, that might rupture cases but an SCS with a full weight bolt is pretty heavy.

My money is on bad ammo.

I would carefully examine the bolt lugs before firing again, maybe get it magnafluxed to see if there are any surface cracks.

 

[awpk03s]

Thanks... a quick internet search yielded Aromics own FAQ page where they discuss this problem and potential... maybe that’s it after all.


http://atomicammunition.com/FAQ.html


Maybe good money to get my bolt inspected I guess. I have a magnaflux shop close by I can quote.

 

[7mmFanBoy]

measure shoulder against other fired maybe slightly out of battery. with that split neck / shoulder have to bad brass . I once turned the neck too far on my bolt gun my case heads started to blow out

 

[redneckbmxer24]

Well it’s certainly atomic lol.

Looks like out of battery or lack of case support to me. Change to SCS is a major clue but it could still be the ammo. If the headspace is shorter than that of the ammo is could certainly be doing it but that’s highly unlikely.

Regardless I certainly wouldn’t recommend shooting that gun or that ammo without figuring out for certain what’s causing it or having it professionally inspected.

 

[SubOptimal]

I don't have a calibrated eyeball to tell very much from the pictures.
Case head separation without signs of severe over pressure (although those primers look pretty flat) would get me asking "once fired in what?".
Only time I have seen that was when I was shooting some multi-time fired Federal brass that had been neck sized only a couple too many times and in a different rifle from which the original firings occurred.
I would get out the calipers and start checking all that brass and get out the head space gauges and check your chamber to be sure about head space in your rifle. Quite possibly a chamber cast and measurement could be in order.
That cracked case is particularly disconcerting. Are you sure the brass is only once fired? It could be a die setting was off and bumped the shoulder way to far back.

 

[awpk03s]

I finally have some resolution to this issue.

I reached out to both Atomic and CMP regarding this, and got separate, and different replies from each.

Atomic, for their part spelled out all of the testing, load development, etc to convey that the loading was believed to be safe. They also said that CMP ordered the ammo to be in once fired military brass, so that’s how they made it. They also shared with me a warning about surplus ammunition, and how it can degrade and corrode over time, and there is always some fallout from surplus rounds. Of course I know that, and when I buy ammo cans of actual military surplus, I sort for rejects. All of these rounds looked as bright and perfect as the day they were born, but anyway. Atomic offered for me to ship the unused ammo back to them on my dime, and they would send me new ammo back at a ratio of 1 new round for every 2 that I returned.

CMP, on the other hand, offered a far better resolution to me as a customer... they sent me FedEx labels to return the ammo to them, and they sent me a check for a full refund of my complete original purchase price.

 

[awpk03s]

As for the rifle itself, I used the funds from CMP to address any potential concerns with the rifle hardware...

The S&W barrel extension is proprietary, and you can’t just replace the bolt, as a standard DPMS pattern bolt will not fit properly (either a complete BCG, nor just a DPMS pattern bolt inside of the S&W carrier).

So I ordered up a Toolcraft nitride BCG, and a Criterion M118LR chamber .308 barrel, 20” length and Rifle gas.

Pretty much the only things S&W on the rifle now are the forged receivers and receiver extension and that’s it.

Next time I get a bit of spare time I will take it out again, zero the scope, and tune gas and JP recoil spring/buffers to optimal results.

 

[padom]

From all the info I have and been told and read you cannot replace the barrel on your S&W MP10. They are proprietary. There was a thread on here a while back where a guy's S&W MP10 barrel split/cracked due to running it in 3 gun matches and getting mud or whatever in the barrel which cause over pressure and the barrel to split. He spoke to S&W who informed him that they do not sell barrels for replacement, they will not replace his barrel but offered him a brand new MP10 at what they called a "discounted" price which was actually more than he paid for the gun.

Keep us posted, because I was pretty sure you cant replace a barrel on a S&W MP10......

 

[awpk03s]

I know that you can not replace the barrel by itself - you must also replace the BCG at the same time with a matching DPMS pattern bolt. At least that’s what I gathered from my research... I will check headspace before going any further with the new bolt and barrel. Worst case I can plumb it all in an Aero M5 upper receiver and mount it on the lower.

 

[awpk03s]

Had I known enough and done the research before hand when I bought this gun 6 years ago (was a rookie on AR10s at the time) I probably would have went a different direction and bought a different gun.

Glad to have the opportunity to keep on learning though. Hope this bolt/barrel combo is the trick and leaves me with a more versatile platform.

 

[LawnMM]

Make sure the bolt is in battery when closed, and locked. The SCS comes with a spacer in case there isn't sufficient forward pressure to close and lock.

I'm not sure even replacing the BCG will work on the Smith depending on the length of the proprietary Smith BCG. The DPMS carrier may be too short/long/whatever.

 

[padom]

As cheap as receivers are today I'd throw that new barrel/bcg into an Aero M5 or similar and move on. You know it will work.

Have 308 headspace gauges? I wouldnt fire that s&w Frankengun without checking with go/no go first....

 

[awpk03s]

I do have a set of headspace gages on the way so I can properly check. And you’re right, if something’s not correct then I know I can just swap the assembly to an Aero upper and be done with it.

 

[awpk03s]

I received the Go/No-Go .308 Win headspace gages today. With its new Criterion barrel and Toolcraft (DPMS pattern) BCG, the rifle passed on both gages.

With any luck I will get some time soon to send some rounds through it and close the loop.

Also, received my refund check from CMP today!

 

[awpk03s]

One last update for anyone who was following the S&W rebarrel aspect of this.

I went out and shot the rifle in its new configuration yesterday (Critetion barrel, Toolcraft BCG). Rifle performed and functioned flawlessly. I shot a few boxes of PPU 168gr and a box of Hornady 150gr SP at 100 yards. Printed a 0.75” (5) shot group with the Hornady soft points, and a 0.5” (5) shot group with the PPU.

When I have more time, I need to still tune the gas and SCS. I was running the SCS with 1 tungsten, 2 steel weights and the medium weight spring. The PPU case heads had some heavy ejector smear (Hornadys were perfect), so I fired the rest of the ammo I had turning the gas block closed a few clicks, until the brass was just dribbling out (at about 2:30) but still had the same smears. So maybe I need to experiment more with the JP spring pack (already have one) and different weights (have those too). Or maybe the PPU brass just sucks.

Either way, I’ll get that figured out. But the rifle functions reliably and barrel shoots great.

 

[RifleBlog]

Thank you for coming back and updating the thread, I’m sure future folks googling for barrel replacements for their M&P10 will appreciate your willingness to take one for the team.

 

[hangunnr]

https://www.snipershide.com/shooting/threads/lr308-failure-to-feed.214109/

OP, take alook at the link above.
I experienced the exact same types of blown case heads just prior to my rifle finally refusing to feed.
The ejector retaining pin was worn causing the ejector to protrude ever so slightly.

Pull your bolt apart and look for worn, irregular parts. There likely isn’t anything wrong with your ammo.