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Thread: AR10 overpressure signs with factory ammo

  1. #1
    Boolit Master
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    AR10 overpressure signs with factory ammo

    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.

  2. #2
    Boolit Master lefty o's Avatar
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    go/no-go gauges would tell you everything you need to know about the barrel/bolt.

  3. #3
    Boolit Master

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    What length barrel?

  4. #4
    Boolit Master

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    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.
    Last edited by country gent; 12-04-2017 at 09:34 PM. Reason: oops

  5. #5
    Boolit Master
    vzerone's Avatar
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    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-part...prod69784.aspx

  6. #6
    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..

  7. #7
    Boolit Master





    bruce drake's Avatar
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    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...t-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.
    I Cast my Boolits, Therefore I am Happy.
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  8. #8
    Bruce is correct on target barrels. one of the Norwegian target barrels i have actually comes out to .3065

  9. #9
    Banned - Charles1990/Eldon/Happy Warrior/Red Jackson/Henry VIII/Mr Humble
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    You need an adjustable gas block.

  10. #10
    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..
    Last edited by Mauser 98K; 12-05-2017 at 05:03 AM.

  11. #11
    Boolit Master gnostic's Avatar
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    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.

  12. #12
    Boolit Master
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    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.

  13. #13
    Boolit Master

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    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.

  14. #14
    Boolit Master Nobade's Avatar
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    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.

  15. #15
    Boolit Master
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    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.

  16. #16
    Boolit Master popper's Avatar
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    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.
    Last edited by popper; 12-11-2017 at 02:14 PM.
    Whatever!

  17. #17
    Boolit Master
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    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.
    Last edited by AggieEE; 12-11-2017 at 01:08 PM. Reason: font color

  18. #18
    Boolit Master
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    Meant to have everything after "dependable" turn back to black. Tried editing and it doesn't seem to work.

  19. #19
    Boolit Master
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    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.

    Click image for larger version. 

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  20. #20
    Boolit Bub
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    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.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check