AR10 overpressure signs with factory ammo

[AggieEE]

Put together a AR10 in .308 Win. DPMS upper and bolt group. Barrel is a 5 grove from Brownell's, I think Pac-Nor made it for Brownell's. Barrel is smooth, when I cleaned it after install, a 30 cal patch on a 30 cal jag was a tight fit. Using Federal American Eagle AE308D 150 gr FMJBT I'm getting over pressure signs, blowed out primers, pierced primers, ejector marks on the brass. I have some LC93 Match I'm going to try tomorrow. Neither the AE nor the LC showed rifling marks on chambered rounds when extracted. Did show marks from getting into chamber. No bright line around the case head indicating excessive headspace. I didn't check to see if the AE bullet was being set back by chambering, will do that tonight. The LC same mag length cartridge before and after. I'll also check to see if the case mouths of the fired cases will accept a bullet easy.

I'm pulling my hair out on this one guys. I don't want to believe that Federal let overpressure shells out of the factory nor that Brownell's sold a bad barrel. I guess the LC's will tell the tale tomorrow. In the mean time does anybody have any ideas? Thanks guys.

[lefty o]

go/no-go gauges would tell you everything you need to know about the barrel/bolt.

[seaboltm]

What length barrel?

[country gent]

Was this barrel a drop in with the barrel extension fitted? Or was it a barrel that needed to be fitted and headspace set ? What leade throat does it have? A short throat or steep leade angle can increase pressures. My Match M1a Has a short chamber ( 155 sierra Palma at slightly under mag length touches rifling) with this M852 LC match with the 168 showed flattened primers during leg matches.
Do you know who made this barrel for brownells? Kriegers and Obermeyers were normally countoured and left .050 under to time and fit to the receivers. Given the basics of what you said I'm suspecting a tight bore and short throat leade angle issue. On the 5 groove I'm suspecting Krieger, Pac Nor, or lilja made

I would check neck dia of the chamber compared to loaded round neck dia first. Then slug the bore to see what groove and bore measure. more so on groove.

[vzerone]

Brownells certainly sells barrel blanks, but I'd say all of their AR barrels are finished ready to go. With that said I'm pretty sure this is the barrel the gentleman has.

https://www.brownells.com/rifle-parts/barrel-parts/rifle-barrels/308-ar-barrels-prod69784.aspx

[Mauser 98K]

slug that bugger and get an idea of the barrel diameter. if it is in tolerance then that is not it.. look at the firing pin and see if it has a rounded smooth end on it. if it has sharp edges this can pop primers.. if the chamber is short and the cases are allowed to jam into the chamber then the brass will not open up like it should and will jam against the chamber walls and will not release the bullet causing high pressure in normal loads.. due to production tolerances of different manufacturers i do not take the term "Drop In" at face value. it might say ready to go or no gunsmithing needed but i have found the opposite to be true 90% of the time. id check my head space with a gauge.. it does not take much to muck it up, about 0.005-0.010 too short to cause problems..

[bruce drake]

Have you slugged the barrel to ensure that is in spec for a .308 caliber barrel? I've known some Palma and Benchrest rifles be extremely tight on their specs. To the point they are often .299"/.307" on their bore and groove measurements of their rifling. http://www.practicalmachinist.com/vb/gunsmithing/308-tight-bore-256684/

Also, a Chamber-Cast will give you a copy of your barrel's throat and identify many of the chambering issues that "country gent" mentions in his post.

[Mauser 98K]

Bruce is correct on target barrels. one of the Norwegian target barrels i have actually comes out to .3065

[lucifers]

You need an adjustable gas block.

[Mauser 98K]

the gas system would not account for the pierced primers. it could account for the ejector marks on the case if too much gas was flowing causing the bolt to slam backwards harder but that would be all.

but AggieEE. if your getting high pressure signs then id trace that down by checking my rifle tolerances b4 i fired any more ammo just to be safe. if it does happen to be the chamber or the bore and your getting enough pressure to blow out primers then you could very well detonate your rifle the next time your fired it, especially if the ammo you fired next was a hotter load..

[gnostic]

You might chamber and eject a test round, to see if the bullet is touching the lands. I've had over pressure symptoms and blown primers from seating the bullet touching the lands.

[AggieEE]

vzerone is correct, that is the barrel I have 20". The loaded rounds chamber and eject easily. I was thinking about the bore dia. I'll slug it when I get a chance. The fired brass will accept a bullet easily but the mouth of the case had a little "snap" going over the bullet. I've seen this with other rifles in different calibers. Near as I can tell the factory brass is 2.005" unfired. The fired length is variable. Seems almost like soft brass. Not having the right tools I can't check brass hardness. Looking down the bore with Mk1 eyeballs the leade does look short but perspective? These 150 gr have a very short length of full dia bullet sticking out of the case. The LC93 has a much longer length of full dia outside of the case and they chamber and extract without leaving rifling marks on the bullet so not that short of a throat. I'll let you know what I find with the slug.

[country gent]

Also there is a difference in the throats of 7.62 Nato and 308 win. This can cause pressures to rise when 7,62 is fired in 308 chambers. I seen a Browning blr locked up with 7.62 nato surpluss at the club one day. The gentleman was using it to "pre Zero" the rifle for his hunting ammo. He didn't blow any primers that I could see but all were very flat. The one in the locked up firearm couldn't be viewed.
I also agree that further firing before finding the issue could be dangerous. Check headspace, slug bore, inspect chamber leade and throat, Also check bolt face for worn surface on face, pits and chips. Try chambering a round by hand with rifle opened up and spring removed, just lightly push on carrier to seat round and close bolt feeling for drag tight areas. A bore scope may tell you a lot. as a chamber cast can.

[Nobade]

I see that frequently with AR-10s. It's usually because the gas port is too big and they start to open while still pressured up. Might try an adjustable gas block to see if you can tone it down a bit. Set it up so it will just lock back on the last shot and it will work a lot better.

[vzerone]

I can recommend a test to see if your gas port is too large. Remove your gas block and just fire one round in it. Of course you will have to manually extract the round with there being no gas, but you can observe your fired case and primer and tell right away if it's a gas problem or other problem. Mind you now there will be high pressure hot gas coming out of that gas port hole!!!!!!!!!!!!!!!!! So be safe about that!!!! I've test fired some AR's that way with no problems. If you do this let us know how the case looks.

Shooting cast in my AR 10's and various bolt rifles I have in 308W there is a very small bullet seating depth difference with the bullet being seated deeper in the 308W rifles. That's one of the differences between the NATO chamber and standard 308W chamber, a little bit more freebore in the NATO. I also have a M1A that has the NATO chamber and there is no seating differences between it and the AR 10's I have. Interesting note I can swap fired brass between the AR 10's and the M1A and they fit one another, but will not fit any of the fired cases in the bolt 308W rifles. The fired brass will easily fit the AR 10's and M1A that I have.

[popper]

I agree with nobade. Also check HS and put a bullet in the muzzle to check bore dia. See a lot of 223 brass with torn rims and ejector dents. Cci primers work in my ar10.
Edit: do NOT check fit with LIVE rounds!! Do you have and USE a case gauge? Must have for AR10. Check fired case length vs factory. You appear to be firing factory only ammo, so you have to use the gauge to check HS.

[AggieEE]

After thawing out from an unexpected snowfall I slugged the barrel. Since it was a 5 grove and I don't have the proper v-block to measure it I used the highly accurate and dependable let's spin it in the mic and try to feel the rotation. I can get .307 to .308 depending on how I turn it. Even if .307 it shouldn't push pressures too high unless the load was close to the line anyway. Next step is do a pound cast of the throat but I think everything will be OK there. If I get a chance I'll put some of the AE through my bolt gun and see what the primers look like.

[AggieEE]

Meant to have everything after "dependable" turn back to black. Tried editing and it doesn't seem to work.

[vzerone]

AggieEE, I'll tell you how to mic that slug. Take flat strip of metal say about 3/8 inch wide and about 2 inches long and you want it to be at least 3/16th inch thick. First bend in exactly half to form a V. Next file a nice flat on the outside of where the point is and plumb to the V. Here's how you use it. First mic a jacketed bullet like the 30 calibers you are using. Then mike it with the V tool in place on the mic with the flat you filed against the stationary anvil and put your jacketed bullet in there re-micing it. Substract the two figures your got and the number you get is the number you use for a base to subtract from bullets you mic in the future such as your slug you made from your barrel. Example: Say your jacketed bullet is .308 and when you use the V tool you get .328. You subtract two numbers and you get a difference of .02. Next mic the slug you made using the V tool and subtract that .02 from the reading and you will then have the diameter of your slug. When you place the slug in the V make sure each side of the V touches the raised groove on your slug. Here's a pic of the V tool.

209314

[egg250]

Assuming the bore, headspace and such all check out and you have taken care of any overgas issues. DPMS LR308 bolts tend to have a *slightly oversized firing pin channel. This can be the cause of pierced and blown out primers. The firing pin needs to almost seal the bolt face but have enough clearance so it doesn't stick. Too much clearance allows the primer cup to flow into the gap and cause a hole or the primer to be pulled out of the pocket on extraction.

Typical military loads likely wouldn't have any issues but the heavier loads can show this problem.

[AggieEE]

Thanks Vzerone. I'll give it a try.

[308Jeff]

Subscribing to this to see how it works out.

[vzerone]

Thanks Vzerone. I'll give it a try.

I came up with that because of all the S&W's I have and they are 5 groove. That is until I bought a V mic. Hope it works for you and hope you understood my crummy instructions.

[AggieEE]

Quick update, I tried vzerone's bent angle. Seems to work but my difference between bullet dia and bullet and "V" changes with bullet dia. Which tells me there is a curve in the bends but beggars can't be choosy. Using a .308 bullet then checking my slug I get .308 for my barrel dia. so far so good. It was suggested that the firing pin was pulling out primers. I checked one of the cases that was missing a primer. I could drop a primer into the pocket so the pocket was expanded. I don't think pulling a primer under pressure would cause the primer pocket to expand, would it? I'll do a throat pound cast when I get a chance just to finish off all of the suggestions. I'm starting to feel that that ammo is just a little warm, ok hot, I still need to try it in my bolt gun just to double check. But, one gun's safe load is too hot in another so ???

[vzerone]

I wouldn't think the primer pocket would get expanded after the primer is out. What's happening is the pressure is very high, and being a cup, the primer seals off the back end of the case and the pressure pushes the walls of the primer not only against the bolt face, but against the walls of the primer pocket, thus the reason the primers pop out. Yes sounds like you have some very hot ammo. I hope you didn't shoot all of it or throw the box away because if you can prove it was hot you can contact the manufacturer of the ammo which in your case is Federal. First check that throat.

[tomme boy]

too much gas and the bolt is unlocking too early. Typical AR10. Very common problem. Get a adjustable gas block and be done with it.

[John 242]

too much gas and the bolt is unlocking too early. Typical AR10. Very common problem. Get a adjustable gas block and be done with it.

Does over-gassing cause primer pockets to expand to the point that primers fall out and new primers to drop into the pocket without resistance?
If so, would you please explain how that happens?

This from the OP-

I checked one of the cases that was missing a primer. I could drop a primer into the pocket so the pocket was expanded.

[vzerone]

Over gassing does not cause primer pockets to swell. Over gassing does cause the primer to severly flatten giving a false indication of an over pressure round.

[John 242]

Over gassing does not cause primer pockets to swell. Over gassing does cause the primer to severly flatten giving a false indication of an over pressure round.

Any other signs, like maybe peening to the corners of the locking lugs, maybe failures to extract and/or damage to the rims of the case, resulting from the case sticking in the chamber, caused by higher than normal pressures?

I seem to recall that higher pressures can cause all of the above.

[akajun]

Gas port has nothing to do with this, if anything an oversized port would theoretically reduce pressures.
Sounds like you either have a tight chamber , tight headspace, or short throat. Do you have a set of hs gauges?? See if it will accept a go.
Next use your stony point/ Hornady oal gauge and measure the throatvwithbthe bullet your using.
Last what chamber is on that barrel? I have a 308 match reamer from ptg that will show signs of over pressure with moderate loads. I now use either an obermyer or m118 reamer on my gas guns, both shoot the 150-175gr matchkings really well.
Btw may want to try some different ammo, including some 762x51 surplus. While 556 is higher pressure than 223, the reverse is true for 308 I’ve found. If your chamber is cut for 762 that might explain your issues.
Let me know what you find, we can get this worked out.

[vzerone]

Gas port has nothing to do with this, if anything an oversized port would theoretically reduce pressures.
Sounds like you either have a tight chamber , tight headspace, or short throat. Do you have a set of hs gauges?? See if it will accept a go.
Next use your stony point/ Hornady oal gauge and measure the throatvwithbthe bullet your using.
Last what chamber is on that barrel? I have a 308 match reamer from ptg that will show signs of over pressure with moderate loads. I now use either an obermyer or m118 reamer on my gas guns, both shoot the 150-175gr matchkings really well.
Btw may want to try some different ammo, including some 762x51 surplus. While 556 is higher pressure than 223, the reverse is true for 308 I’ve found. If your chamber is cut for 762 that might explain your issues.
Let me know what you find, we can get this worked out.

An oversized gas port does not reduce pressure. The larger the gas port the higher the pressure and the more volume of gas going back to the bolt/carrier group. With short barrels the gas port is brought more near the breech end of the barrel and it's smaller in size because the gas pressure is higher the closer to the breech you get. On some short barreled pistol AR's that are still using a DI system the gas port can be very near the chamber because a pistol cartridge doesn't have enough gas volume and pressure to operate the action.

[popper]

Does over-gassing cause primer pockets to expand to the point that primers fall out and new primers to drop into the pocket without resistance?
Not normal but can happen. Usually, PP is expanded as primer cup is expanded by high pressure. Ok. Now what happens if primer is pushed out a tad, then crammed back in? Kind of a 'top-hat' on the cup. Big reason H.S. and gas are important. Did OP ever check ammo with case gauge? And see if it 'fits' his rifle?

[vzerone]

Not normal but can happen. Usually, PP is expanded as primer cup is expanded by high pressure. Ok. Now what happens if primer is pushed out a tad, then crammed back in? Kind of a 'top-hat' on the cup. Big reason H.S. and gas are important. Did OP ever check ammo with case gauge? And see if it 'fits' his rifle?

Over gassing can never expand a primer pocket that the case is no good anymore. Only a very very high chamber pressure can do that. Please don't perpetuate a new myth. Over gassing will cause a harsh action cycling and also starts to open the action earlier then designed to. Just switch a swollen primer pocket over to a bolt action rifle that has no gas system. See my point?

[akajun]

I think you are confusing gas pressure from firing, which is what causes the powder to burn and create pressure to push the bullet down the bore , with increased gas flow/ volume from the gas port which is used to cycle the action. The pressure is the same from an oversized port or an undersized port, it’s the volume that’s different . If anything, bleeding off more gas rim an oversized port would bleed off pressure, but I believe that the powder has all burned up by the time the bullet passes the gas port on an ar10 anyway so peak pressure has already been achieved.
He has a chamber / throat/ hs issue , evidence is the popped and expanded primers, that’s handloafing 101 for signs of overpressure.
If he was ripping off rims or throwing cases 30 feet then I’d say he’s over gassed.

[vzerone]

I think you are confusing gas pressure from firing, which is what causes the powder to burn and create pressure to push the bullet down the bore , with increased gas flow/ volume from the gas port which is used to cycle the action. The pressure is the same from an oversized port or an undersized port, it’s the volume that’s different . If anything, bleeding off more gas rim an oversized port would bleed off pressure, but I believe that the powder has all burned up by the time the bullet passes the gas port on an ar10 anyway so peak pressure has already been achieved.
He has a chamber / throat/ hs issue , evidence is the popped and expanded primers, that’s handloafing 101 for signs of overpressure.
If he was ripping off rims or throwing cases 30 feet then I’d say he’s over gassed.

I'm not confusing anything sir. I know very well how the AR and it's DI system works. I believe I mentioned both volume and pressure. The pressure going back to the bolt/carrier group is quite high, more then many would believe. I'm trying to get across the point his problem isn't the gas system and the gas system has no correlation with swollen primer pockets.

The OP was suppose to fired the other ammo which I don't believe he has. If that shows high pressure signs it just about leaves out the ammo.

[popper]

Over gassing can never expand a primer pocket Correct, I stated that improperly. IMHO he has a H.S. problem (or bad ammo), the overgassing just doesn't help - he got ejector dents and bad primers too. He needs to get a case gauge and check HIS rifle's H.S. with the factory ammo. Overgassing just slams the case head into the bolt - not good. Once fired case normally doesn't show a 'ring' of weakened case head. Extreme pressure will extrude brass around the ejector and FP hole. If he just taps any 308 jacketed into the muzzle he can easily measure bore and groove. Loose barrel nut can be the problem also, this appears to be a self assembled rifle.

[vzerone]

Over gassing can never expand a primer pocket Correct, I stated that improperly. IMHO he has a H.S. problem (or bad ammo), the overgassing just doesn't help - he got ejector dents and bad primers too. He needs to get a case gauge and check HIS rifle's H.S. with the factory ammo. Overgassing just slams the case head into the bolt - not good. Once fired case normally doesn't show a 'ring' of weakened case head. Extreme pressure will extrude brass around the ejector and FP hole. If he just taps any 308 jacketed into the muzzle he can easily measure bore and groove. Loose barrel nut can be the problem also, this appears to be a self assembled rifle.

We're good popper. I like ya pardner! Yeah I think it was David Tubbs that came out with his carrier weight systems to delay that bolt opening action. The best way to have done it was with an adjustable gas valve because adding weight to the bolt/carrier group just slams the hell out of your barrel extension and the group itself.

He did a lead slug of the barrel popper. I drew him that V tool so he could mic it as the barrel is a five groove.

Aggie please do a test firing of the other ammo so we can see if it does the same thing with it.

[John 242]

Over gassing can never expand a primer pocket that the case is no good anymore.

This is what I was thinking as well and thank you for clearing that up.


Aggie please do a test firing of the other ammo so we can see if it does the same thing with it.

If I had the gun in front of me, I would first check headspace, just because, then I would test fire it with another brand of (good quality) ammo, as you suggest.

I don't "think" headspace has anything to do with this, but it makes sense to work analytically and scratch that off the list as well. And then ... ?

I have seem problems lately with guys firing Wolfe steel case .223s in several different AR-15 type guns. Cases stick in the chamber and the extractor overides the rim, tearing it up. They will also have flattened primers. The guns headspace fine and brass cased ammo extracts and ejects fine. I can speculate as to the cause of the problem, but simply changing ammo "fixes" it.

[6.5/06]

Forgive me if it has been mentioned(didn't see it), but the first handloads I fired in my AR 10 had similar problems. (even though several grs. below max charge). It was the new Fed brass I used, had soft case heads. Seems Fed has a reputation for this. They said to ship it back to th

[AggieEE]

Short update again. Took me a while to touch base with my gunsmith, work weather etc, and the bolt would lock up an a no-go gauge. Next step will be to talk to Brownell's about the barrel and see where it goes from there. I don't see how excessive head space would expand primer pockets but fix one problem and test again. I may have two issues at the same time or not. Will update as I learn things.

[popper]

Torn rims are from extractor, head dents from the ejector pin. Both can be caused by early bolt unlock. Haven't had a CCI LRP (#200) primer pierced yet. I don't use 308 Fed brass (or ammo), toss any I find. Now that you got the rifle checked out, get some decent Win/Rem hunting ammo and try it.

[sawinredneck]

Torn rims are from extractor, head dents from the ejector pin. Both can be caused by early bolt unlock. Haven't had a CCI LRP (#200) primer pierced yet. I don't use 308 Fed brass (or ammo), toss any I find. Now that you got the rifle checked out, get some decent Win/Rem hunting ammo and try it.
What problems did you have with Fedral loads?

[AggieEE]

swainredneck, Original problem was expanded primer pockets, lost primers etc. It was fed red label 150gr fmj so it should be a good solid load. Having the bolt close on a no go gauge for a new barrel is pretty much a no no so I need to talk to Brownell's and see what we can come up with. So the ammo may just be a symptom and not the problem or I got lucky and have two problems at once.

[AggieEE]

OK this should be the final update, at least I'm calling things good. I got the replacement barrel from Brownell's also went ahead and got a set of headspace gauges and a new DPMS style bolt. Got everything put back together and no-go will not close in chamber. So far so good. Finally got a chance to shoot it, working sucks but so does not working, tried some LC match, don't remember what year. Normal ejection, primer a little flat but still in the case and showing some radius to the edge of the primer. Same results with some mil-surplus M80 ball. Fired one round of Fed. Pressure signs big time. Tried one round in my M1A and still showed a little warm. Extractor rub on the case head, primer had almost no radius to it.

I broke down 3 rounds of the Fed and weighted the bullets and powder. The bullets went from 149.7 to about 150.2 gr. the powder charge was right around 45 gr IIRC but also had about 0.5 gr spread or a little better. The powder looked ok, no smell or red powder. It looked about like 3F black powder as far as grain size and color. About the size of salt or a little coarser. I know our powders and factory powders are two different animals but according to the Sierra book the closest powder would be H335. I don't know what H335 looks like but since I have 38 rounds I'm just going to break them down fertilize the grass with the powder and reload with a known good load. Lesson learned is try one round first in the rifle and check the case even though it's factory ammo. Thanks for everybody's input.

[Nobade]

Glad to hear that you got it fixed!