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View Full Version : Head space or ammo? Something I'm not thinking of?


Ferrum
04-19-2012, 8:50 PM
I have been having trouble with .308 and don't know if it's an ammo issue or a head space issue with the barrel. .308 is an AR style platform that I built, which I put a DPMS 24" SS Bull Barrel on (complete upper). I have been mixing ammo to try to get a some kind of repeatability (including reloads) to no avail. About every 60-80 rounds, the round fires and the rifle turns into a smoking mess. When this happens I separate the upper and find the case with the primer blown out, and the case compressed. This makes for a fun time at the range....

Any ideas?

Pictures attached of the aftermath...

kurac
04-19-2012, 8:54 PM
ouch, that looks like its firing out of battery or there is way too much headspace

russ69
04-19-2012, 9:04 PM
Use hard military type primers and be sure they are fully seated. Have a good smith look at that rifle.

SWalt
04-19-2012, 10:03 PM
hmmmmm......looks like a belted case! were these reloads, remanufactured or manufactured rounds? reloads for a semi auto need to fully resized and not neck sized. headspace is critical and needs to be a few thousandth less to chamber properly. blown primers are usually from over pressure, load too hot, or can be sloppy primer pockets. wrong powder with too fast or too slow burn rates could cause over pressure too with semi autos gas systems. lots of things can being going on here. the cases being compressed like that seems like the gases escaping out of the blown primer are being forced back around the outside of the case through the barrel. which would mean your bolt is fully closed and sealed so the only route the gases have are around the outside of the case out though the barrel. strange...thought most barrels had the means to vent those gases safely. i'm not an expert so i wouldn't shoot it until you get it figured out. not worth losing fingers and eyes if things really go wrong.

Ferrum
04-19-2012, 11:01 PM
Rounds have a mix of reloads and factory. Been firing about everything to see if I can get it to repeat. When fired the round actually hits down range on target, so it's building enough pressure to move the round.

@Swalt, what you are saying about the pressure building out the primer makes sense... That would definetly compress the case.

TheHammerOfTruth
04-19-2012, 11:06 PM
OP,have you put a gauge in that chamber yet? PLease check it before someone gets hurt.

NorCalK9.com
04-19-2012, 11:30 PM
Wow, i'm not a reloader so maybe its common, but ive never seen primers blown like that before.
Good luck op.

SWalt
04-20-2012, 1:00 AM
Were you trying to get your rifle to shoot accurately but its throwing rounds all over the place? Is that what you meant by trying to getting it to repeat? Are the rounds you are using just a mixed bag of reloads and factory( manufactured or remanufactured) ammo? If it is just a bunch of mixed rounds in a box, separated them all by head stamp and bullet type. Might be a good idea to get a go/ no go gauge to SAAMI specs for .308 and gauge each round to see if they all are within spec. Its not unheard of ammo not being to spec by manufacturing flaws. I would set the reloads aside and not use them. If all the rounds are to spec, then it can very well be your rifle. Call the manufacturer and tell them whats going on. The chamber/throat maybe wasn't milled right and is out of spec. Many things can cause a round not to chamber properly which can cause a blown primer and cause it to not shoot accurately. Visually inspect everything to see if something looks wrong. Rough machining, hair line cracks, gouging, etc. The smallest details can mean things. We don't think about it much but a semi auto is a finely tuned mechanism which operates at extremely high speed in a certain sequence of events. Details matter. Its worth saying again, don't shoot it until you got it figured out.

sonnyt650
04-20-2012, 7:44 AM
I don't think it's a chambering issue where on functioning AR-pattern bolts and carriers the firing pin doesn't break the plane of the bolt face unless the bolt lugs have engaged the barrel extension lugs. Instead I'll say that's some pretty bad overpressure right there, and if the stamp on the bottom is still visible try to determine which batch of ammo is causing the problem.

Looked at them myself: Lake City '07 -- reloads?

Bhobbs
04-20-2012, 8:02 AM
I would stop shooting that rifle. It's an accident waiting to happen. Get some gauges to check head space and the cases. It may be either of those or a botched chamber.

sonnyt650
04-20-2012, 8:30 AM
So what do the factory rounds look like? It occurred to me that if it's not an issue with the amount and type of powder there could be a problem with the cases themselves. They should have zero lubricant of any kind on them, otherwise the case when expanded doesn't grip the chamber walls and you get pressure signs as the full force of the expanding gasses drives the case like a piston into the bolt.

Ferrum
04-20-2012, 9:17 AM
Factory rounds look exatly the same. The two rounds posted are reloads, they are just the last ones I ran thru the gun. I did check the head space when I put this together, and was within tolerances (haven't rechecked it since this started, will do that this weekend). I put in a call to DPMS and I am waiting on a call back from a tech.

gvbsat
04-20-2012, 9:46 AM
Thats some serious over pressure. It's not the gun IMO. And, the flash hole, it looks like it has been opened up a bit. Doing so will cause high pressures with charges not exceeding load data and most defiantly blow the primer out.

gvbsat
04-20-2012, 9:49 AM
Never mind, I didnt see the first picture. Didnt notice it until just now, it does look like its firing out of battery

SWalt
04-21-2012, 8:23 AM
So...the 2 pictured are reloads. Are the reloads the only ones that have blown primers and get crushed? Or do the factory rounds have the same problems?

triaged
04-21-2012, 8:41 AM
It could be a out of spec. throat. I had a similar problem with my CMMG 5.56 upper and sent it back. Throat is what they said and replaced the whole upper for me. Send it back to the manufacturer or take it to a gunsmith who can make a cast of the chamber.

edit: didn't look at the pix first. :eeek: that is bad! Mine was nothing like that but the primer were loose in the case. Dint shoot it any more till it gets fixed!

30Cal
04-21-2012, 9:02 AM
Take it to a smith. Somethings badly wrong.

I think sonnyt is got the right idea; it's tough to get the rifle to fire out of battery when you pull the trigger... It's done it with factory ammo. Therefor, I think either the bolt is unlocking way too soon or there's something up with the barrel that's causing a major pressure problem. I'm thinking like an undersized bore or something really bad with the throat, or the lugs on the bolt or barrel recesses not timed correctly.

I'm curious how the bolt lugs look?

On a side note, I'd have serious concerns about the integrity of the bolt at this point. They handle pressure fine under normal situations, but you're wayyy outside that. I would consider the bolt unservicible even if you find and fix the issue.

Ferrum
02-11-2013, 9:09 AM
I know it's been awile, but I just finally was able to address this over the last month. Long and the short.... My bolt was damaged due to over pressured rounds. I will post a picture of the bolt as soon as I can... The over pressure caused a hair line fracture to the bolt and messed up the gas rings.

I had a gunsmith check everything else out, replace the BCG, check the head space, and it's good to go.

Becasue of the over pressure issue, I took a part the some of the rounds I was using and believe that I found the culprits... I believe that these were casued by Miwall reloads. All of the factory rounds that I looked at, and the rounds that I reloaded, were very consistent. The Miwall rounds were from 46 to 50 grains of powder, which is a huge spread. I am not sure what powder was being used, but can guess that this is very problamatic. I did find one round that was 52.8g which i'm guessing, is more than enough to damage the bolt.

Now the Miwalls are out for me...

paul0660
02-11-2013, 9:14 AM
Thanks for clearing it up. I remember seeing this thread last year and being baffled.

30Cal
02-11-2013, 1:51 PM
Interesting.... Glad you got it worked out and had the rifle looked over.

ar15barrels
02-11-2013, 6:38 PM
I have been mixing ammo to try to get a some kind of repeatability (including reloads) to no avail.
About every 60-80 rounds, the round fires and the rifle turns into a smoking mess.
When this happens I separate the upper and find the case with the primer blown out, and the case compressed.

Pictures attached of the aftermath...

For anyone elae that bumps into this thread in the future, the first post contains a perfect example of what the cases from 85,000 psi pressure loads look like.

As the maximum pressure is 55,000 psi for the 308, those loads were extremely over-pressure loads.

dust feeder
02-11-2013, 6:51 PM
Could someone explain exactly what's happening with these overpowered rounds to cause all the damage. I can obviously see the primers are blown out. But with the bolt closed shouldnt the bolt be holding them in place and supporting the back of the case? How is there room for the primer to go anywhere?

What is happening to mess up the rim of the case so badly?

ar15barrels
02-11-2013, 7:25 PM
Could someone explain exactly what's happening with these overpowered rounds to cause all the damage. I can obviously see the primers are blown out. But with the bolt closed shouldnt the bolt be holding them in place and supporting the back of the case? How is there room for the primer to go anywhere?

What is happening to mess up the rim of the case so badly?

So think of the cartridge case as a "pressure gasket".
The gasket is rated to hold about 60,000 psi.
When you exceed that pressure, the brass becomes elastic and will "flow" into areas with less support.
There is a chamfer at the mouth of the chamber.
Under normal pope rating pressures, the pressure gasket is strong enough to hold it's shape.
When pressure increases, the gasket stretches to fill in the unsupported areas.
There is extra clearance around the diameter of the case head.
The case stretches from the center outwards to fill that area.
That leaves the primer pocket loose enough for the primer to just fall out after you eject the case from the action.
The bolt face has a hole drilled in it on the side.
That hole contains the ejector and ejector spring.
Under extreme pressures the case can actually extrude down into that hole.
Again, under normal pressures, the pressure gasket does not stretch into those unsupported areas.
The extractor cut in the side of the bolt face is another unsupported area.
In the pictures above, you can also see that the pressure gasket expanded into the unsupported area where the spring loaded extractor lives.

Think about play dough.
Imagine you had a flat piece of wood with a 1" hole drilled in it.
Now make a piece of play dough about the size of a hamburger patty.
Set the play dough on top of the wood, covering the hole.
Under its own weight, the play dough is strong enough that it does NOT sag and fall into the hole.
How take the palm of your hand and apply 20 lbs of pressure on top of the play dough.
The play dough is no longer strong enough to support that force and some play dough will extrude down into the 1" hole.
When you peel the patty back up off the piece of wood, you will see a dome shaped piece of playdough that extruded down into the hole.

HK Dave
02-11-2013, 7:37 PM
Randall has the best examples. lol

Ask him about his cupcake example sometime.

dust feeder
02-11-2013, 8:30 PM
Thanks Randall. That's a great explanation. I hadn't realized that the case head is actually stretching outward allowing the primer to fall out after being ejected. I thought it was actually just blowing the primer out. I recently saw the after effects of a guy who tried to chamber a second flu d while one was already chambered in his mosin nagant and ignited the chAmbered round with the tip of the second round. That Actually blew the primer out of the open bolt. I figured something similar was happening to these primers.

I also didn't realize that those tabs we are seeing are actually parts of the case extruded into available orifices. Thought it was just getting chewed up I. The action. That's really incredible! Thanks again for the explanation. I'm just a casual shooter so its always good to learn a little more about the technical side of these things

ar15barrels
02-11-2013, 9:06 PM
I also didn't realize that those tabs we are seeing are actually parts of the case extruded into available orifices.

Even if you completely ignore the obvious extruded tabs sticking out of the case head at strange spots, check out the LC 07 stampings on the case head.
You have likely seen them on other pieces of brass yourself, but the stampings were MUCH deeper.
Notice on the high pressure case that the stampings have been pushed back out of the case head so that the stampings are very shallow now.