762 gas gun consistantly blowing primers?

[David Pennington]

I've got a well known, well regarded AR pattern 762 gas gun that is having significant functioning issues.

It runs GI M80 Ball fine but blows about 20% of the primers from commercial (SAMMI) spec ammunition. The rifle consistantly blows primers with:

-Federal 180gr GMM
-Federal 165gr Tactical Bonded
-Hornady 155gr TAP

These blown primers cause doubles feeds via ejection failures. The rifle extracts fine but the ejector doesn't kick out the spent brass. It appears that the gas flow from the popping primers is mucking up the ejector and inhibiting ejection even when the primers don't pop.

This occurs both unsuppressed and suppressed using a Surefire 762K.

GI M118 SB (173gr Ball) runs fine unsuppressed but double feeds on nearly every round surppressed.

I could buy that it's an ammo issue if it were only happening with one type of ammo, <span style="text-decoration: underline">or even one brand of ammo</span>, but it happens with every commercial .308 load tried. The manufacturer says they have never seen this issue before and they are currently working on the rifle. The tech suggested that the gas port was too large but I, and several other knowledgable gun guys, can't see how that would effect chamber pressure. Bolt carrier velocity, sure... but not chamber pressure.

The manufacturer replaced the barrel with a new barrel with a smaller gas port and they are reporting that it still blew a primer on 1 of 5 Federal 165gr Tactical Bonded rounds I sent.

Does anyone have any insight into this before I send up a few hundred rounds of $1ea ammuntion for the manufacturer to test?

Thanks.

 

[bustin]

Re: 762 gas gun consistantly blowing primers?

excessive headspace can allow the primer to back out under pressure or if the action opens early(over-gassed) while there is still pressure in the case it can do the same thing.
if you reload you can size the brass to the chamber but if you use factory ammo the only way to fix it is tighten up the headspace.
You could test the early opening by spinning the gas block to cover the port then shoot it and see if it still blows primers.

 

[ORD]

Re: 762 gas gun consistantly blowing primers?

Out of curiosity...is the rifle CHAMBERED in 7.62x51 (7.62 NATO) or .308 Winchester?

 

[jakhamr81]

Re: 762 gas gun consistantly blowing primers?

Military ammo has a crimped primer, that is why it is running better, stick with it. As far as the suppressed issue, you may want to try an adjustable gas block.

 

[Scottmilk9]

Re: 762 gas gun consistantly blowing primers?

try a heavier buffer, could be unlocking too quick creating the pressure issue.

 

[scotharr]

Re: 762 gas gun consistantly blowing primers?

Is it blanking the primers or actually blowing them out of the case? Blanking is usually from excessively large firing pin holes and blowing them out could be from headspace or other issues.

Who is the manufacturer? AR-10s have a long history of problems like these and its recommended to work with a truly experienced manufacturer. I find it hard to believe that any legitimate manufacturer has "never seen this problem."

 

[krecon1]

Re: 762 gas gun consistantly blowing primers?

Here's an email I received a couple years ago from Black Hills when my Mk11 Mod had a couple dropped primers:

After examining the two pieces of brass you returned it appears as though these are what we call "dropped primers". On a "blown" primer we usually see primer pocket expansion with case head expansion as well. These two don't show either of that.

Pressure tests performed on the lot you have at time of manufacture indicated a pressure of 55,277 psi. Max pressure per the Sporting Arms and Ammunition Manufacturers Institute is 62,000 psi. These are well within the pressure range.

From the burr on the case head it appears that you're firing these in a semi-auto weapon. The burr is a result of the bolt beginning to unlock while there's still pressure in the case. As the bolt moves, the pressure that's left pushes the primer out. The use of a suppressor helps hold the pressure in the barrel even longer. I'm sure that if you examine your fired brass, regardless of manufacturer you'll probably see the burrs.

Neither the weapon nor the ammo is at fault. It's just that the semi-auto 308 Win chambered weapons really should use ammunition loaded in brass that has crimped in primers. This usually means mil-spec brass. You may fire hundreds of rounds of commercial without this happening but eventually it will. The loose primer will probably not cause any harm as it'll simply drop out of the weapon. It can, however, fall into the trigger group and cause a malfunction. In a law enforcement encounter this is obviously a bad thing.

If you're using this weapon on duty we'd recommend using ammo with crimped in primers. This and other commercial ammo would be great for practice and training but one cannot afford a jammed weapon in a violent confrontation.

I hope this explains what happened with these rounds.

Sincerely;

Carl Bullock
Customer Service Rep
Black Hills Ammunition

 

[David Pennington]

Re: 762 gas gun consistantly blowing primers?

 

[Hellbender]

Re: 762 gas gun consistantly blowing primers?

Looks like one of two things, either a chambering issue, excessively short throat, too tight chamber, headspace problem, etc.

The BCG on these guns are so heavy, they can actually resize a case to fit a too tight chamber.

2nd....Excessive bolt head "slop" (can't think of the proper term right now), ie, the distance the barrel extension locking lugs are from the face of the barrel can cause this also. It should only be a few thousandths longer than the locking lug length of the bolt.

This problem won't show up with a headspace gauge, but when the bolt shoves the cartridge in the chamber, it actually sets the cartridge case shoulder back. An out of spec (lugs too short) bolt head can also cause this.

 

[bustin]

Re: 762 gas gun consistantly blowing primers?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: David Pennington</div><div class="ubbcode-body">

</div></div>
Those show excessive pressure with brass flowing into the ejector hole and extractor groove,
brass starts to flow around 65000 psi so there is high pressure present.
Extra oil in the bore and chamber left from cleaning can cause high pressure, severe copper or carbon fouling can too.
I still think the rifle is opening early due to an over sized gas port, a carbine gas system will make it worse on a AR10 that's why we only use mid or rifle length systems.
Shoot it in the dark, if flames or sparks fly out of the ejection port it is opening too early and the gas block needs to be sleeved.
The primers aren't flat, it looks like the bolt is pulling away from the cartridge while there is still high pressure in the case. spin the gas block to cover the gas port and shoot it just to see if it still blows primers. If it does not then you know the rifle is over-gassed and how to fix it.

 

[BigDTexas68]

Re: 762 gas gun consistantly blowing primers?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ORD</div><div class="ubbcode-body">Out of curiosity...is the rifle CHAMBERED in 7.62x51 (7.62 NATO) or .308 Winchester? </div></div>

I just had a AR 308 built with a 7.62x51 chamber. I have only fired 125 rounds with and without a can and used mil surplus ammo. I am also running an adjustable gas tube. So far zero issues. Match 308 chambers just seem to be giving ARs some problems. I think with tight tolerances everything else about the gun has to be just right for it to function.

 

[KevinB-KAC]

Re: 762 gas gun consistantly blowing primers?

What we noticed in the SR-25EMC / M110 Carbine program was that 7.62 NATO/.308 does better in a rifle length gas system than midlength with the 16" guns. You should then not need an adjustable gas system for suppressed/unsuppressed usage.

Semi-Auto guns like crimped primers. But I've shot a lot without, and if the gas system is right, and the bolt and chamber are correct it should not cause major issues - unless your getting bullet setback that is increasing pressure.

With what I see above, I think it also may be a case neck tension issue with the ammo.

Dave *sorry I forgot to mention that on the phone.
Also please hit me with an email, I need to mooch some stuff...

 

[sinister]

Re: 762 gas gun consistantly blowing primers?

Typical with the bolt carrier group starting extraction while the cases are still obturated to the chamber walls (ESPECIALLY with the suppressor).

Try Slash's Extra-heavy 10-ounce AR-10 XH buffer AND the heavy tungsten slug Tubb Carrier Weight System to help delay primary extraction.

180s should NOT be pushing out primers. Military match ammo does NOT have crimped primers.

 

[KevinB-KAC]

Re: 762 gas gun consistantly blowing primers?

I think he has a Carbine.
The Slash rifle buffer will not work.

 

[David Pennington]

Re: 762 gas gun consistantly blowing primers?

Kevin is right, it is a 16" carbine with a collapsable stock.

Also, I tested for bullet set back and it is not an issue. The primers also pop when single loaded.

 

[KevinB-KAC]

Re: 762 gas gun consistantly blowing primers?

Dave, I'll get you a good deal on one of ours

Are the 165gr a Poly tip?

 

[sinister]

Re: 762 gas gun consistantly blowing primers?

Slash makes an Extra-Heavy for 16-inch carbines as well. I doubled on a 16-inch AR-10 carbine using both heavy buffer and CWS.

7.62 direct-gas autoloaders are great tools that have received unfair reputations for reliability problems since many designers assumed dimensions for 5.56 directly translate to 7.62-sized packages.

Dave Tubb did wonderful work with the SR-25 when he was using it at Camp Perry, and Steve Thompson at ADCO in Ohio was offering a slightly longer gas tube and rail to lengthen carrier dwell time.

It looks like KAC's use of the full-length gas system on the EM carbine "Broke the code" and has fixed the SR-25K's shortcomings.

 

[Grump]

Re: 762 gas gun consistantly blowing primers?

Sure that's not just soft brass?

I would suggest a mathematical model "almost" reality check by running QuickLOAD for those cases' water capacity (don't forget to correctly input the fired case length) and all likely powders at made-up charge weights to match your chrono'd velocities.

I've run some references to published loads from old magazine articles on .308 loads *with* chamber pressures and found pretty good correlations between QL and the real-life observations...taking into account the oddities of CUP vs. Piezo PSI transducer readings *for that caliber*. There are true comparison problems with military pressure readings because some go at the throat area instead of mid case body like everyone else uses, IIRC.

That's my best suggestion, since no published reports of case head hardness (I *think* brass uses a form of Brinnel and NOT Rockwell, scale starts too high) exist that I know of, and none of us is likely to be able to measure it on a recognized scale anyway...

But I really look sideways at those ejector hole scrub marks. That usually means bad ju-ju, even though I have found it on dozens of once-fired FEDERAL .308 factory load brass over the years.

 

[HillbillyfromAL]

Re: 762 gas gun consistantly blowing primers?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ORD</div><div class="ubbcode-body">Out of curiosity...is the rifle CHAMBERED in 7.62x51 (7.62 NATO) or .308 Winchester?</div></div>

What's the difference? Alot of ammo I used in the Army was marked as 7.62x51 NATO/ .308win

 

[DP425]

Re: 762 gas gun consistantly blowing primers?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: HillbillyfromAL</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ORD</div><div class="ubbcode-body">Out of curiosity...is the rifle CHAMBERED in 7.62x51 (7.62 NATO) or .308 Winchester?</div></div>

What's the difference? Alot of ammo I used in the Army was marked as 7.62x51 NATO/ .308win </div></div>

Chamber dimensions primarily. The two chambers are not exact copies. Everyone believes .223 and 5.56mm are different but .308 and 7.62 are identical. Couldn't be further from the truth. As I recall, pressure standards are nearly identical (unlike .223/5.56) but actual dimensions are not. Shouldn't be to the point of causing danger in swapping between the two but it could have accuracy issues and potentially function issues.

As I recall, the 7.62 chamber is a bit larger in almost every dimension and the .308 may have a marginally higher pressure spec than NATO does.

I don't know that it would give you this many problems though- the specs aren't THAT far appart

 

[Grump]

Re: 762 gas gun consistantly blowing primers?

I just cannot imagine how NATO headspace "longer" by something like the thickness of a sheet of paper can possibly allow a primer to fall out, or a primer pocket to expand. IME, all it does is induce excessive case stretch on firing, leading to case separations (usually 3/8 to 1/2 inch above the head, where the case wall stops tapering and switches to almost parallel) is as few as three firings.

Point of interest--the 5.56 NATO chamber difference from .223 SAAMI is mainly in the throat or leade--important mainly with M193 and probably M855 bullets AFIK. In .308/7.62 NATO, it's the chamber max and "no-go" and "field" (one of the two doesn't even HAVE one of those two standards) dimensions that are different. I believe it is Fulton Armory which reports having NEVER found ANY factory ammo, military or commercial, which was NOT loaded to the same minimum AMMO dimensions.

Me, I've never been able to locate SAAMI and NATO *cartridge* dimensions with *both* min and max values for either one, not to mention both. Thus, I cannot realistically evaluate the real-world dimensions of maximum tolerance stack of min ammo in max chambers.

But these two things I know--size your ammo too short and brass dies prematurely, and size the brass not enough in a semiauto and you're a tiny bit more likely to get a slam-fire in an M1A, especially with spherical powders and high primers.

 

[Grump]

Re: 762 gas gun consistantly blowing primers?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: David Pennington</div><div class="ubbcode-body">Kevin is right, it is a 16" carbine with a collapsable stock.

Also, I tested for bullet set back and it is not an issue. The primers also pop when single loaded. </div></div>

Not quite complete enough to eliminate all bullet setback possibilities. That variable could probably be eliminated IF your single-loaded chambered rounds, when ejected unfired, still measure the same OAL as before chambering.

It's a throat thing.

If you know your load's "jump" to the lands, which means there *is* a jump, then the rounds should not be any shorter.

But since the rifle is being worked on...no can do, right?

And I think that if you had a 7mm-08 bore with a .308 chamber and throat, your percentage of loose primer pockets would be more like at least 80%. Don't laugh too hard--I've heard credible reports of that happening a few times over the years.