Why is my extractor deforming rims?

W

wchartz

Full Member
Full Member
Minuteman
Nov 8, 2011
343
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McKenzie, TN
#1
I have a DPMS Gen2 in .308. It has a Superlative adjustable gas block. The action spring and buffer are the originals. The gun has about 400 rounds on it. The problem I am about to describe occurred in the original 16" barrel and now in a Rock Creek RTR 18" barrel installed by Craddock Precision. I am trying to work up some loads using Hornady 168 gr. ELDM bullets, LC and Federal brass, Winchester LR primers. All brass is once fired, full length resized and trimmed to minimum length. The powder charges listed are the spread from highest to lowest load charge. I am not showing the charges in between the highest and lowest. The loads are as follows; 8208 XBR ,42.5-39. CFE 223, 46.7-42.5. Varget, 43.5-39.5. The following comments apply to all of the above loads. At the heaviest charges I see brass extruded into the ejector hole and sheared off by the rotating bolt, extremely flattened and cratered primers, one popped primer, one pierced primer and on most cases the rim is bent and sometimes fractured by the extractor. With the lightest charges, the primers are still badly flattened but not as badly cratered. there are no ejector swipes but the bent and sometimes fractured rims are actually a little worse. As a control I shot some Federal Gold Medal Match 168 gr. They showed slightly flattened and slightly cratered primers plus bent and fractured rims. The chronographed velocity of the Federal was 2581 f.p.s. The only reload I chronoed was the 42.5/8208 XBR at 2641 f.p.s. I tried the gas block settings at 1.5 and 6 revolutions from the closed position. With the hottest loads the ejection was consistent at the 3 o'clock position on both settings. With the lightest loads I used the 6 revolution setting and ejection varied from 2 to 3 o' clock. All loads ejected and locked the bolt open on the last round.
I am a highly experienced reloader (60+years) but not very experienced in AR platforms.
I would appreciate any fact based information that would help me with this problem.
 
#2
Would you mind posted a picture or two? Are you able to adjust the gas block closed so the bolt doesn't cycle?

It kinda sounds like the extraction is taking place before the bullet leaves the barrel.
 
#4
The action will not open with the gas block fully closed. The bolt will not lock back when the adjustment screw is opened up 10 revolutions. The case heads look the same either way except there is no rim damage when the bolt does not open.
I have a rifle lenght gas tube. The action tube is 6.875", the spring is 10.75" and the buffer is 3.75 oz.
In the photos, #---572 has my hottest load on the left and my mildest load on the right. Photo # ---573 of the rim damage looks the same on all loads, including the Federal GM Match. The primers on the GM Match #---574 also look like my lightest reloads which is the case on the right side of #---572. The hottest and mildest for each of the powders I have listed in my original post look about the same on the primers and rim. The brass does not extrude into the ejector hole on the lighter loads. On the ---572 picture, right case, you can see the raised rim section between the 8 and W on the case head.
I am not shooting suppressed at this time but I want to be able to in the future.
Thank you for your interest.
 

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#6
wchartz said:
The action will not open with the gas block fully closed. The bolt will not lock back when the adjustment screw is opened up 10 revolutions. The case heads look the same either way except there is no rim damage when the bolt does not open.
I have a rifle lenght gas tube. The action tube is 6.875", the spring is 10.75" and the buffer is 3.75 oz.
In the photos, #---572 has my hottest load on the left and my mildest load on the right. Photo # ---573 of the rim damage looks the same on all loads, including the Federal GM Match. The primers on the GM Match #---574 also look like my lightest reloads which is the case on the right side of #---572. The hottest and mildest for each of the powders I have listed in my original post look about the same on the primers and rim. The brass does not extrude into the ejector hole on the lighter loads. On the ---572 picture, right case, you can see the raised rim section between the 8 and W on the case head.
I am not shooting suppressed at this time but I want to be able to in the future.
Thank you for your interest.
Click to expand...

Please post some pictures of the rifle, barrel, BCG, buffer. All of the small receiver 308 guns have reduced dwell and carrier/buffer weight which makes them more sensitive to ammunition selection. The slower the powder and higher the pressure, the worse they get.
 
#9
Appears to be a pressure problem, high pressure on some with too much powder for the bullet, and too high port pressure as extraction is starting when pressure is still to high. Lower the pressure at extraction time, ..delayed.
Turn down adjustable gas block setting, less high pressure gas entering the gas tube delaying extraction, or added weight.
Try less gun powder first.
 
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#10
45-90 said:
Appears to be a pressure problem, high pressure on some with too much powder for the bullet, and too high port pressure as extraction is starting when pressure is still to high. Lower the pressure at extraction time, ..delayed.
Turn down adjustable gas block setting, less high pressure gas entering the gas tube delaying extraction, or added weight.
Try less gun powder first.
Click to expand...
The primers on my reloads do indicate excessive pressure. I am puzzled as to why, according to the Hornady manual, moderate charges of the 3 commonly used powders are showing such pressure signs. The rim damage seems to be a different issue. That damage also occurs on Federal GM match. I have adjusted the gas setting from "restricted" to the point of failure to function to "bypass" to the point of failure to function. The primers and rim damage looks the same at the different settings.
ShtrRdy said:
It sounds like the original configuration was a factory DPMS and had the same problem? If so, have you ever talked to them about the problem?
Click to expand...
No. I got the rifle about 8 years ago when DPMSwas an independant company. I doubt that they would want to help me since the barrel and gas block are not original. I need to clarify. On my original gun I did not have the rim damage problems but it did show a lot of pressure signs on the primers with moderate loads. It had a mid length gas tube and the RTR barrel is a rifle length tube.
 
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#11
Drop the powder charge 2 grains and see if the rim damage goes away, or improves.
Doesn't matter what the manual says, at this point.
Then open the gas to function.
 
#12
that can happen even at less than max pressures/over pressures. i have/had a 308win ar that did that due to gas port size being wrong. bolt speed being to fast and unlocking to early can do that. shoot 1 w/empty mag and see if bolt holds open, if so try 1 in mag and chamber another, shoot and see if it picks up the next round.
 
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