Overgassing?


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wgungho
April 4, 2013, 06:42 PM
I have a recently aquired Stag Mod 1. I'm the second owner and the first owner put <100 rounds through the weapon. I have fired several types of ammo through the weapon with only one presenting a problem. Using Privi, headstamped PPU-11, I get persistent failure to extract. This ammo is marked M193, 55 gr., BT. The extractor actually tears part of the rim from the casing. The casing remains completely in the chamber. A very gentle tap with a cleaning rod, and the casing drops free. It isn't "stuck" in the chamber at all. There are no marks or gouging on the spent casings, just the damaged rim. The chamber is clean and clear of any visible burrs. This rifle has M-4 feed ramps. Overgassing would seem the obvious cause to me, but my last Stoner weapon was issued to me in 1968, and it was a POS. I demanded and got back my M-14! I have limited experience with AR systems, but I do understand the mechanics.
Suggestions from other sources have included replacing the buffer with an H2 and heavier spring, using an O-ring on the extractor spring, using a heavier extractor spring, replacing the bolt carrier with a heavier, mil-spec piece, and polishing the chamber (which is chromed). Again, I have cleaned the chamber well. I assumed the possibility that laquered ammo was used, and cleaned for that, also.
The obvious solution would be to use other ammo. Regretfully, the huge majority of my .223/5.56 is this Privi, PPU-11. I've tried ammo from three different lot numbers, with the same results. No one that I've talked to has had anything bad to say about this ammo.
All observations are appreciated.

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rcmodel
April 4, 2013, 09:33 PM
I have heard of no specific problems with that ammo.

Of course the only thing to do now is, scrounge up some other brand of U.S. made ammo and see if the problem persists.

If it does?

Return the STAG to STAG for repair.
If it's over-gassed to the point of ripping case rims off?
The gas port is too big and the barrel will have to be replaced to fix it.

rc

CLP
April 4, 2013, 09:38 PM
Well, you did the first thing. Cleaned the chamber. I don't think there would be much benefit from polishing the chamber if it's chromed. The next cheapest thing would be dealing with the extractor and associate parts. Look closely at the extractor to ensure it's intact and that there aren't any burrs or anything on it. Most factory extractor springs all have an insert and o-ring. You could start by trying to replace those. They're cheap. If the extractor itself is suspect, then try replacing it. A flat spring chrome silicon buffer spring would be the next cheapest thing to do, followed by a heavier buffer. It's interesting how some rifles don't like particular brands/lots of ammo. I've never had a problem with Prvi. I've felt their brass was a little soft compared to LC and Rem. Also, ensure your brass isn't dusty/dirty. Wipe it down with a rag.

rcmodel
April 4, 2013, 09:49 PM
The extractor actually tears part of the rim from the casing.Seems to me like the extractor and spring are doing all they can be expected to do.

rc

CLP
April 4, 2013, 09:59 PM
But he's fired several types of ammo and only one is causing problems. He can send it back, but with current backlogs I'd be surprised if he saw it back anytime in the next several months. I'd change out the extractor spring and ensure there was an o-ring on it. It sounds like there may not have been. That may just be the tinkerer in me, or my impatient side, or both. But one things for sure, he's unlikely to find any other ammo unless he wants to pay a hefty premium.

CLP
April 4, 2013, 10:01 PM
He should also inspect to ensure the bolt lugs are all intact. A chipped or broken lug would cause asymmetric torque on the case upon extraction and might cause the described problem. There's an Armalite technical note on a similar issue (though re: the AR-10).

rcmodel
April 4, 2013, 10:12 PM
I'd change out the extractor spring and ensure there was an o-ring on it.O-rings are not necessary if it's tearing the rim off.
IMO: O-rings aren't necessary if it isn't.
I've never needed them in properly running Colt AR's in the last 43 years anyway.


I didn't notice he said he had no problems with several other brands of ammo.

But that right there is what I like to call a CLUE it's the ammo, not the gun!!

rc

CLP
April 4, 2013, 10:24 PM
You bring up an interesting point. It's recommended that you do not use an O-ring with the chrome silicon ejector springs. I don't know why exactly, they don't feel any more compressible than the run of the mill ej. springs, but that's completely subjective.

Jim K
April 4, 2013, 10:39 PM
"But he's fired several types of ammo and only one is causing problems."

Duh. Forget a detailed analysis of the gun, just stop using that ammo. Problem solved.

(FWIW, the condition described is due to high residual pressure, which in turn is almost always caused by the wrong kind of powder. It's the ammo, not the gun.)

Jim

CLP
April 4, 2013, 11:37 PM
"But he's fired several types of ammo and only one is causing problems."

Duh. Forget a detailed analysis of the gun, just stop using that ammo. Problem solved.

(FWIW, the condition described is due to high residual pressure, which in turn is almost always caused by the wrong kind of powder. It's the ammo, not the gun.)

Jim
Easier said that done in today's market. Trying to help the guy find a practical solution.

rcmodel
April 5, 2013, 12:16 AM
Trying to help the guy find a practical solution.If the ammo powder pressure curve is wrong?

The only practical solution is an Adjustable Gas Block so he can dial down the gas port pressure to work with the over-pressure out of spec ammo.

rc

mtrmn
April 5, 2013, 11:49 AM
Had this same issue with several guns that were marked 5.56 on the bbl but the chambers WERE NOT cut to full 5.56 specs in the throat area. Tried heavier buffers/springs etc, which improved it, but the final solution was a Christensen reamer which removed a surprising amount of metal. I ran this reamer through 7 or 8 barrels and the only one that passed the test was my Colt 20". The problem with ripping rims off etc when I used 5.56 ammo disappeared once the so-called 5.56 chambers were actually re-cut to proper specs. Bought the reamer from Michiguns.com

45_auto
April 5, 2013, 12:58 PM
Hope those weren't chrome-lined barrels - if they were, the chambers aren't anymore!

mtrmn
April 5, 2013, 01:57 PM
Hope those weren't chrome-lined barrels - if they were, the chambers aren't anymore!
They weren't. Chrome lining is of no significance to me for my purposes so I didn't hold out for that option during the last Obama panic of '08--I took whatever I could get/afford. They also weren't 5.56 even though they were marked as such. They came from 2 different sources, one of which is fairly well-respected in the parts kit business. I won't mention their names since this was 4 years ago and I feel the problems may have been corrected since then. But I will put forth my firm belief that there are most likely a LOT of barrels out there marked 5.56 that are not cut to proper 5.56 specs.

wgungho
April 5, 2013, 05:39 PM
Guys,
I've read and appreciate all the replies. Not using the ammo causing the problem is an obvious answer to the problem, but the Privi I have is the mainstay of my ammo supply. I've looked all over the net and in many different firearm sites, and no one is having a problem with this ammo--specifically Privi headstamped PPU-11 5.56x45. And I mean NO negative reviews. It has to be something in my rifle, and I'm boneheaded enough to keep digging until I figure it out. I have ordered a Wolf extractor spring, I'm looking for an H2 buffer, and I'm cleaning the chamber yet again. Can anyone tell me what size o-ring fits over the extractor spring?
Thanks for your replies!
Tom

SilentScream
April 5, 2013, 07:36 PM
You won't be finding that o-ring at the local hardware store. However I feel you're going in the wrong direction on this. Adding tension to the extractor will simply result in more of the case being ripped off. Obviously it's the ammo, it could be a pressure curve issue, it could also be out of spec. (diameter too small) case heads. That being said, no amount of after market widgets will solve the problem. STOPPING THE USE OF SAID AMMO WILL SOLVE THE PROBLEM.

CLP
April 5, 2013, 09:59 PM
http://www.brownells.com/rifle-parts/extractor-parts/extractor-hardware/o-rings/ar-15-m16-extractor-donut--prod32504.aspx?avs|Make_3=AR-15/M4


OP, is all of your PPU ammo from the same lot?

wgungho
April 5, 2013, 11:22 PM
It's all PPU-11, but three different lot numbers. Same problem with all three lots.

mtrmn
April 6, 2013, 12:00 AM
These type problems are the main reason somebody came up with mid-length gas systems. The gas tube is longer and the gas port farther up the barrel which will serve to delay unlocking the bolt long enough for chamber pressures to drop.

The 16" bbl carbine-length gas system combined with 5.56 pressure levels, lighter-weight AR15 bolt carriers, normal-weight buffers/springs, and possibly out-of-spec chamber dimensions will combine to tear the rims off cases. The same case will be easily removed from the chamber with a cleaning rod and show no signs of being stuck in the chamber.

This is a matter of timing and high chamber pressure when the bolt unlocks and the extractor tries to pull the fired case from the chamber. The 5.56 chamber has a much longer throat funneling the bullet into the rifling. This delays and spreads out the pressure spike that occurs when the moving bullet meets resistance as it engages the rifling. The bullet starts moving and then when it meets resistance it actually stops again until the burning gases build up enough pressure to push it on down the bbl.

If the chamber is a normal 223 profile the throat or funnel will be very short, and the leading angle into the rifling will be much more abrupt. This causes the pressures to spike sooner and a lot higher than the 5.56 chamber would see.
When the bullet passes the gas port, the gas travels thru the gas tube to the bolt where it unlocks the bolt. The bolt starts moving back and when it tries to extract the case, the case is still expanded and welded to the chamber walls due to the excessive pressure-which SHOULD have already dropped enough to allow extraction. The bolt is going to cycle regardless of whether or not it can bring the empty case with it. Since the case is gripping the cylinder walls so tightly, the extractor rips off the rim and goes on its merry way.

You can change timing and sufficiently slow down the bolt unlocking by
1- adding weight to the BCG or buffer and a heavier buffer spring
2-by moving the gas port farther up the barrel as in a midlength system
3- by controlling the amount of gas that gets to the bolt with an adjustable gas block--this is the most desirable but you may have to adjust back up with weaker ammo.
4-Most likely it will require a combination of the above measures with iffy results--you'll just have to experiment
You will have to use a single round loaded in a mag to test. Run the heaviest buffer etc that will reliably lock the bolt open on the last shot from a magazine. This will have to be tested using the WEAKEST ammo you intend to use.

IF your chamber really is out-of-spec for 5.56, the above tweaks will only do so much good and your gun will still be subjected to excessive pressures with 5.56 ammo. The cure for this is:
1- properly ream the chamber to 5.56
2- re-barrel to a known good 5.56 chamber
3- Just don't shoot 5.56 ammo

The O-ring and heavier extractor spring will NOT help with this problem-they are intended to fix an extractor that slips off the rim of the case, not one that takes the rim with it.

http://www.m-guns.com/tools.php
There is a gage and a reamer on this page along with some good info. The gage is not cheap, and the reamer would pay for a new barrel but in my case it saved numerous barrels, so I consider it worth the money. If you happen to live in Louisiana I could allow you to use my reamer if you wish, but it won't leave my property. You'd have to pay me a visit.

I have written this in an effort to truly help you out and it is based on my own hard-earned lessons in WECSOG. I have tried to be as clear and honest as I can. YMMV.

Jim K
April 7, 2013, 04:38 PM
True, to a point. In this case, there is no special concern about pressure UNTIL the bullet passes the gas port and allows gas to enter the tube. Then, in the time it takes the gas to move back and impinge on the bolt carrier, and the bolt carrier to unlock the bolt,
the bullet leaves the barrel and the pressure drops. For a very short time there will be some pressure, called residual pressure, remaining in the case. Normally, that is no problem and in fact will assist extraction by pushing the case out of the chamber. But if that residual pressure is too high, the case will continue to adhere to the walls of the chamber and the extractor will tear through the case rim. Such high residual pressure is associated with the wrong powder, or with something in the gun that allows the action to open too soon.

It is not clear how an "out of spec" chamber could cause that, unless the chamber is rough so that even low pressure will keep the case from extracting. If that is the case, then obviously "rechambering" will correct the problem by reaming and smoothing the chamber walls, not by altering the chamber contours.

Jim

CLP
April 7, 2013, 09:20 PM
He can get a stiffer buffer spring like here: http://www.brownells.com/rifle-parts/receiver-amp-action-parts/action-springs/ar-15-car-15-ar-style-308-chrome-silicon-springs-prod22336.aspx?avs|Manufacturer_1=SUPERIOR%20SHOOTING

But he's not likely to find any FA bolt carriers any time soon. He could use this to help keep the bolt locked up a bit longer: http://www.brownells.com/rifle-parts/bolt-parts/bolt-carrier-parts/bolt-carrier-hardware/ar-15-carrier-weight-system-prod12720.aspx?avs|Manufacturer_1=SUPERIOR%20SHOOTING

mtrmn
April 8, 2013, 08:49 AM
True, to a point. In this case, there is no special concern about pressure UNTIL the bullet passes the gas port and allows gas to enter the tube. Then, in the time it takes the gas to move back and impinge on the bolt carrier, and the bolt carrier to unlock the bolt,
the bullet leaves the barrel and the pressure drops. For a very short time there will be some pressure, called residual pressure, remaining in the case. Normally, that is no problem and in fact will assist extraction by pushing the case out of the chamber. But if that residual pressure is too high, the case will continue to adhere to the walls of the chamber and the extractor will tear through the case rim. Such high residual pressure is associated with the wrong powder, or with something in the gun that allows the action to open too soon.

It is not clear how an "out of spec" chamber could cause that, unless the chamber is rough so that even low pressure will keep the case from extracting. If that is the case, then obviously "rechambering" will correct the problem by reaming and smoothing the chamber walls, not by altering the chamber contours.

Jim
The difference between 5.56 and .223 is more in the THROAT. Actual chambers are basically identical. The THROAT is where the already high-pressure 5.56 ammo will give a pressure spike that will keep the case welded in the chamber for an instant too long and give the extractor just enough time to rip the rim off.
Adding weight to carrier/buffer will help this as will a stronger spring. But you can get too heavy and the gun may short stroke with weaker ammo.

Jim K
April 8, 2013, 07:11 PM
I guess I have a hard time seeing that. A tight chamber neck can cause a significant pressure spike, but once the bullet enters the throat it is well underway and the pressure spike should be over. And it should be over by the time the bullet makes its way past the gas port, and the gas moves back through the tube, and the cam dwell in the bolt carrier imposes its delay, plus the turning bolt head causes its delay -- I just don't see a pressure spike at the throat lasting that long and the pressure still being high enough to keep the case "glued" to the chamber walls.

But strange things do happen inside a rifle.

Jim

mtrmn
April 8, 2013, 09:58 PM
I guess I have a hard time seeing that. A tight chamber neck can cause a significant pressure spike, but once the bullet enters the throat it is well underway and the pressure spike should be over. And it should be over by the time the bullet makes its way past the gas port, and the gas moves back through the tube, and the cam dwell in the bolt carrier imposes its delay, plus the turning bolt head causes its delay -- I just don't see a pressure spike at the throat lasting that long and the pressure still being high enough to keep the case "glued" to the chamber walls.

But strange things do happen inside a rifle.

Jim

Please don't take my diagnosis as being argumentative in any way. I AM just a one-tooth knuckledragging backwoods redneck mechanic after all.;) I'm just putting forth what worked (on more than 1 gun) in my experience in an effort to help. My explanation of WHY I think it worked may be totally wrong.

I realize also that 1 simple question remains to be asked of the OP:
We know the ammo giving trouble was 5.56, were any of the other non-offenders 5.56 or were they .223? Brands and loadings of all the ammo would be helpful.

wgungho
April 9, 2013, 07:48 PM
I've tried several different rounds through the Stag. These include Lake City M855 and M193, Monarch .223 55gr soft point, Hornaday .223 60gr sofrpoint interlock, Independence 5.56 55gr fmj, Atlanta Arms and Ammo .223 62 gr. VMax and, of course, the Privi, of which I'm blessed with an overabundant supply. My problem with the Privi is that if I don't use it, I have to give it back. My "rich uncle" has a Spikes that absolutely loves the Privi. All of the above had failures to extract, with varying percentages of failures per rounds shot. Both Lake City rounds were like the Privi, with 75%+ failures Most of the different types I fired 10 shots each. None of the casings were "stuck" in the chamber. I did not clean the chamber during the outing, but the rifle was not dirty after being fired. The rifle was cleaned to start with, (I'm a Marine, so it was clean) with special attention paid to the chamber. I lubed the weapon with Mobil 1, wetting down the BCG, leaving the chamber, bore and bolt face dry. I do not have a bore light, so it was difficult to light up the chamber, but I could see no machine marks or flaws in the chroming.
The one item that stands out is that through three outings the rifle fired Remmy PMC without a single failure, period. Last trip I slow fired a mag, quickly followed by dumping an entire mag, with no problem. I made no attempt to shoot for accuracy, just firiing for function, and I killed a trophy pine tree. In three excursions, the PMC has been flawless.
It is my intent to order an H2 carbine buffer and a new spring, hoping that one or the other, or both, will help with the problem. I'm also trying to get in touch with a Stag rep to see what they have to say.

mtrmn
April 9, 2013, 09:51 PM
So most, if not all, failures to extract were with 5.56 loads if I'm reading that right.
If my info is correct, the only difference between 5.56 and .223 ammo is that 5.56 is a good bit hotter. This is because in order to get the .223 round adopted by the military back in the day they had to meet a standard that the regular .223 round just couldn't reach. It seems there was some requirement that it had to have enough energy left at 500 meters to reliably penetrate a military helmet. So somebody decided to just keep loading it hotter until it would meet that requirement. Thus the 5.56 came into existence.

I know the above story is open to all kinds of arguments and may just be complete fiction, but it sounded believable to me and serves the purpose here in explaining why 5.56 generates higher pressures than .223. Which is why there are so many warnings and dire predictions about using it in .223 chambers. I really don't think it's that dangerous in ARs, just far more prone to malfunctions.

I have ASSUMED all this time that your barrel is marked 5.56, but that's another basic question I have failed to ask. The BARREL has to be marked, not the lower receiver. The marking may be underneath the handguards, and it may not be marked at all.

Mobuck
April 9, 2013, 11:33 PM
Try not to confuse the chamber NECK with the THROAT(leade).
Problem might also be due to slightly softer brass which doesn't "spring back" after the pressure drops. The softer brass will also exacerbate the rim ripping syndrome.

taliv
April 10, 2013, 12:01 AM
i agree with MTRMN

i wouldn't hesitate to use the aforementioned reamer from Ned C if you have one handy. however, calling stag to get them to make it right may be the better choice.

wgungho
April 11, 2013, 09:05 AM
The barrel is indeed marked 5.56, and Stag is very specific that the rifle will shoot both calibers. Plus, I cannot believe that all the types of ammo that I've tried have soft brass. A couple of friends have shot the same group of ammo that I tried and experienced no failures.

I have called Stag, and I'm waiting for a return call from their service/repair people. I've located an H2 buffer and heavier spring, but I want to be sure that using them won't void Stag's most excellent warranty.

mtrmn
April 11, 2013, 05:34 PM
Good deal, maybe Stag will get it 100% for you.

I don't think it will hurt to just TRY the heavier buffer/spring as long as it's not still in the gun when Stag gets their hands on it.

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