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Posted: 3/9/2009 11:57:23 AM EDT
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I just recently built an AR10 and put the first round through it.
I noticed obvious pressure signs right away and stopped firing. I am waiting on a headspace gauge but these rounds are in spec, hand cycle, and are at the minimum end of the powder charges. The brass in the pics are of an unfired round and an extracted piece of fired brass. This load was 41 grains of IMR 4895, Wolf Large Rifle primer, 168 SMK, LC brass trimmed to 2.005", loaded to mag length. The brass has a flat primer and some serious abrasion marks on most of the case. 
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| The reason your primer has backed out on you is because your powder charge is way too light. 21.0 grains of IMR-4895 is close to HALF a normal charge weight. Standard match loading using IMR-4895, Lake City brass, Winchester primers and 168 grain Sierra Match Kings seated at 2.80" is 40.5 grains. This is also considered a maximum or very near maximum load for a gas operated rifle. Try 40.0 grains even, it should work just fine. |
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Something has to be f'ed up to show pressure signs with those low charges.
I load 168 gr A-MAX bullets for my 20" AR-10 using 44.0 grains of Varget with no sign of pressure. Although the load is accurate, my velocity is around 2,500 fps. I get 2,750 from a 150 gr Nosler over 47.5 grains of W748 which also shows no signs of pressure. The hs of my chamber is very generous, but seems to cause no obvious problems other than stretching my brass more than I like. Can you give more details about your build? |
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Looking at your fired case against an unfired it looks like the shoulder is bumped back too far. You need a lyman case gauge to check your reloads or fire some factory rounds. Getting a 308 gauge
right now is difficult but not impossible. You will have definite headspace issues if the should is short. It's hard to tell in pictures but it is the most likely cause. I haven't ever seen a bolt and barrel out of spec on an ar although I guess it could happen. |
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Quoted:
Looking at your fired case against an unfired it looks like the shoulder is bumped back too far. You need a lyman case gauge to check your reloads or fire some factory rounds. Getting a 308 gauge right now is difficult but not impossible. You will have definite headspace issues if the should is short. It's hard to tell in pictures but it is the most likely cause. I haven't ever seen a bolt and barrel out of spec on an ar although I guess it could happen. I would concur, only if the barrel and bolt were both brand new from Armalite, or if the barrel maker supplied the bolt. It would help to know what exactly he has. |
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I don't see the high pressure signs. Your primer may have backed out to be flush with the head surface but that does not necessarily mean over pressure. Your primer dimple looks good. Your primer cup edges are still rounded. Overpressured primer cups will come to almost square edges and some will even come to a "tee" shape, which you can only see after you decap.
The neck looks like it is not sealing hard, because there is some soot back around the outside of the neck. The shoulder looks like it did a good job sealing against the chamber bore. When I shoot really hot loads the neck seals hard. I think you may be a little uptight like I was when I shot my first reloads, in your case probably even more so since you built the AR10. The scuff marks along the outside of the casing can come from the chamber bore imprinting on the casing at firing pressure, also from sliding in and out of the chamber, also from the full body sizing die as well. New rifles often have sharp edges on the lugs of the barrel extension that grate on the brass as it is extracted. Probably the most reliable indicator of running too hot, is to check the girth at the [0.200] reference distance from the casing head surface. The diameter from unfired sized to fired should not change more than .001. For instance, if you start with .4703 dia. at the [.200] distance but end up w with a .4713 or more. Then, definitely stop shooting. |
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I've experienced similar results with my DPMS LR308T. Loads that work fine in my FAL and M1A with no pressure signs, seem to show pressure in my DPMS. A standard accuracy load of 41.5 IMR 4895 and a 168 SMK showed flattened primers in my DPMS but are fine in my M1A and FAL. I'm not sure why, I just reduced the powder charge for the DPMS.
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Thanks for all the responses guys.
The bolt is a brand new Armalite and the barrel is an Armalite 20" chrome lined which was bought on the EE and only had minor break in rounds put through it. The rail is a DD full length and the barrel was tightened correctly with some grease on the threads. This is my first AR-10 and the gas blowback is much more significant than an AR15 so maybe that kind of made me tentative after seeing the brass scrapes and primers flattened more than I am used to so I didn't want to press my luck. I am waiting on a case gauge and headspace gauge so I can get a better idea of what is going on. By using an adhoc method of measuring the fired cases it seems that the shoulder of the case fired in the AR-10 is almost .007" longer then one fired in my Rem 700 bolt. A factory or unfired 7.62 piece of brass is around .001" under my Rem 700 fired case measurements. I know this is not ultra accurate until I get the proper tools but it sounds like you guys may be correct about the shoulder being bumped too far back. It also sounds like you guys may be correct on the bolt unlocking early which would give false readings such as the shoulder being expanded much larger then the chamber actually is. Or I could have a headspace issue which is unlikely but obviously I can't be sure until I measure. |
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On headspace: Read the tech notes #60 & 69 on www.armalite.com. and check out this link:http://www.thegunzone.com/30cal.html
It would not be unusual to have to create a separate stream for reloads of your AR10 from your Remington 700. Here is a commercial .308 Win Power Point 150 grain fired in a SA SOCOM II 16" bbl., chronographed at 2660 fps. I submit the primer cup is flatter than the ones you show. The second frame shows the drill/reamer marks from the Springfield Armory machined chamber embossed into the casing (circumferential) and overlayed by longitudinal marks from forced extraction. Customary dings show up as well. 
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Thanks for the info CCW
I had read the Armalite tech notes but not that webpage. What amazes me is that plenty of people put 308 factory ammo through AR-10s but having a looser chamber is safer then the other way around. I guess I will be resizing for two different firearm headspacing. Worst case I know I can fire the rounds FL sized for my Rem 700 in my AR-10 and my rounds sized for my AR-10 will not chamber in my REM 700. |
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