I say we blame it on the ammo!

This is a fairly new upper that I built with all new parts, except for the barrel.  About 95% of the ammo that I have run through it was Q3131.  I did shoot some Black Hills 75 grain through it, but I did not note what the brass looked like.  I'll try some different ammo next time, and see how it looks.

Ok.  Range report.

Original bolt.  Removed D-Fender.  Extractor freely moves with pressure.   Q3131 ammo.  Five rounds (unknown sequence):
1 - normal
1 - normal (with light extractor marks.
3 - significant extractor scoring (visible brass ridges, able to catch on fingernail)

Next five rounds, same set-up but with commercial .223 reloads (LCC 02, NATO-stamped brass):
5- normal

REPLACED BOLT (new CMT MPI from Bravo Company).  

Extractor tight.  Similar to having a D-fender installed.  Q3131.  Five rounds:
1 - normal
1 - polished ejector mark -- very light extractor marks
1 - polished ejector mark -- heavier extractor marks
2 - deep extractor scoring, similar to problem with other bolt.

Same commercial .223 reloads as above.  
Five rounds: 5 - normal
Eighteen round string:
5 - normal
6 - normal (very light extractor marks)
7 - normal (heavier extractor marks)

REPLACED ORIGINAL BOLT

Same commercial .223 reloads as above.
13-shot string.
4 - normal
9 - light to medium extractor scoring.

Completely UNSCIENTIFIC observations:
1.  Q3131 scoring caused by higher pressures and soft brass.
2.  Original bolt has an extractor that is digging deeper into the brass than the new one (but not by much).
3.  .223 loads do not exhibit scoring unless fired in a string greater than 5 (no more than 5 seconds apart)

Completely UNSCIENTIFIC/UNEDUCATED conclusions:
1.  The problem is primarily caused by the chamber heat increases causing the chamber to "grab and hold" the case in the chamber.
2.  Since the Q3131 is a "hotter" round, the chamber heats up and "grabs" the case sooner than .223.
3.  Even with .223, if shot in a string with no greater than 5 seconds between firing, the chamber heats up sufficiently to "grab and hold" the case.
4.  The Q3131 and LC brass may be softer than other cases, contributing to the problem.

So..., if I've narrowed it down, would a Wolf or MGI spring alleviate the problem because the would cause a greater "dwell" time?  It would seem that messing with the gas pressure would be a nightmare because if you solved it on the high end, you'd create problems on the low end.

Or, finally, is this just a normal condition and I shouldn't worry about it anymore.

So close, it’s the residual pressure in the barrel that is causing the rounds to be more pressure bound to the sidewalls with the hotter loads.  Throw in fouling, and or rather the chamber getting hotter and burning off what CLP does migrate into the chamber to semi clean the fouling from the side walls and allows a more fluid spent case release, and you may likely seen what is casing the occasional marring.

The stronger springs/ heavier buffer will slow down the bolt unlock (carrier movement), and would mast the problems of an over size gas port (read over functioning).  

Now the question that is coming back to blind side you?  Did you try to run the rifle with a standard buffer and standard spring to weed out if the rifle is over functioning (hence the MGI buffer and stronger spring just masking the real problem)?

And by all means, stop loosing sleep over the ordeal.  As you said, you are not going to reload the brass, so what denting/ mar’g to the cases is really not your problem.

Quoted: So close, it’s the residual pressure in the barrel that is causing the rounds to be more pressure bound to the sidewalls with the hotter loads.  Throw in fouling, and or rather the chamber getting hotter and burning off what CLP does migrate into the chamber to semi clean the fouling from the side walls and allows a more fluid spent case release, and you may likely seen what is casing the occasional marring. The stronger springs/ heavier buffer will slow down the bolt unlock (carrier movement), and would mast the problems of an over size gas port (read over functioning).   Now the question that is coming back to blind side you?  Did you try to run the rifle with a standard buffer and standard spring to weed out if the rifle is over functioning (hence the MGI buffer and stronger spring just masking the real problem)? And by all means, stop loosing sleep over the ordeal.  As you said, you are not going to reload the brass, so what denting/ mar’g to the cases is really not your problem.

No.  I haven't tested it with a standard buffer.  The current spring is a standard spring.

Not losing sleep.  Especially after seeing that the new bolt does the same thing and that Triple_D had the same thing, with the same rounds, from the same retailer.  

The primary purpose of the thread was to make sure that I was not seeing signs of a dangerous condition developing.  Second, I wanted to understand why it was happening and third, I wanted to see if there was anything that I needed or could do on my end.  One and two seem to be covered.  I'm still not sure about the third answer -- but I need to re-read the whole thread again.

I have learned a bunch troubleshooting this question.  I really appreciate all the info that everyone has provided.  I'm going to print out this whole thread and put it in my notebook -- and re-read it again and again.

Quoted: My RRA 16" is throwing spent cases with marks on the cartridge rim face that concern me.  The marks dig into the brass slightly in a straight line across the lower 1/4 of the rim face.  Some actually result in small points of brass extending out from the edge (caused by the brass being dug out).  The point of brass is generally in the same line as the rim face mark.   I don't have a camera with enough resolution to show them to you. No other problems -- seems to shoot fine and otherwise extract correctly. I am using an MGI buffer with a standard carbine spring. Any assistance would be appreciated. WW

How do you determine that it is the lower half of the rims face that is getting marked if your looking at ejected brass? Are you using the extractor marks to determine this? If its possible that the marks are actually on the top of the face, it very well could be sharp edges on the bolt lugs causing the marks on the brass... This would happen during the stripping from magazine and feeding process...

Quoted: How do you determine that it is the lower half of the rims face that is getting marked if your looking at ejected brass?

I'm sorry, you are correct.  I don't know whether it is the upper, lower, right quarter, etc.

Are you using the extractor marks to determine this? If its possible that the marks are actually on the top of the face, it very well could be sharp edges on the bolt lugs causing the marks on the brass... This would happen during the stripping from magazine and feeding process...

Hmm...hadn't thought about the lugs.  The mark, however, is pretty much exactly the arc of the extractor face and is consistently located at the expected location from the exjector marks (when they occur -- which is not very often -- like when I used the new bolt).

Quoted: Quoted: How do you determine that it is the lower half of the rims face that is getting marked if your looking at ejected brass? I'm sorry, you are correct.  I don't know whether it is the upper, lower, right quarter, etc. Are you using the extractor marks to determine this? If its possible that the marks are actually on the top of the face, it very well could be sharp edges on the bolt lugs causing the marks on the brass... This would happen during the stripping from magazine and feeding process... Hmm...hadn't thought about the lugs.  The mark, however, is pretty much exactly the arc of the extractor face and is consistently located at the expected location from the exjector marks (when they occur -- which is not very often -- like when I used the new bolt).

Your noticing the mark where the extractor would make contact with the brass? Are you using a WOLF extractor spring or rubber O-ring to support the extractor?  It could be the face of the extractor claw that is sharp and digging into the face of the brass...

Quoted: Your noticing the mark where the extractor would make contact with the brass?

Yes.

Are you using a WOLF extractor spring or rubber O-ring to support the extractor?

Stock spring.  Removed D-Fender...no difference.  Replacement bolt ejector tighter that original bolt.

It could be the face of the extractor claw that is sharp and digging into the face of the brass...

Yep.  That's my thinking ... but on the ejection/extraction phase.  I think that we have established that due to the direction of the scoring (see above).

Basically, the chamber is heating up and "grabbing" the case.  It doesn't do it with low pressure rounds unless shot in a string.  I don't believe that there is anything that can be done about the chamber temperature.  My current thought is that I can help by getting a heavier buffer spring (increase dwell time?).

However, full auto weapons generate far more heat in the chamber that 10 or 15 rounds in a string out of my AR...so...what does their brass look like after a 30-round dump?

I don't see how it could be possible to create that type of mark, in that particular location, while extracting\ejecting...  I'm thinking its occurring while the round is being pushed into the chamber, when the extractor is up against the face of the round, while forcing open the claw, right before it grabs onto the rim...

Quoted: I don't see how it could be possible to create that type of mark, in that particular location, while extracting\ejecting...  I'm thinking its occurring while the round is being pushed into the chamber, when the extractor is up against the face of the round, while forcing open the claw, right before it grabs onto the rim...

From Randall_Raush (above):

The bolt unlocking while the case is still holding on to the chamber walls is what gives you the wipe marks as the bolt smears or swages the brass back into place while unlocking.

hmmm, ok... Then the mark is under the face of the rim? The projectile\bullet side of the lip? Or the primer side?

I went back and found the pictures... Looks to me like its happening when the round is being chambered...

Outlined in red, notice what looks to be a brass stain on the top of the claw? Well at least it looks like its a brass stain to me...  Anyway, the left part of the extractor claw looks to be digging into the brass a tad... The low quality pic might be fooling me but it sure does look like a brass stain...

Quoted: If you are getting marks like these: images.snapfish.com/345%3B%3A62%3B2%7Ffp338%3Enu%3D3256%3E%3B87%3E338%3EWSNRCG%3D32334%3A5262389nu0mrj That is classic over-function combined with soft brass or higher pressures than the brass is hardened for. The brass is flowing back into the extractor and ejector cuts. The overfunction comes in because the bolt is unlocking while high barrel pressure is still holding the case in place. Go back and read my "how it works" post again!

No need to.  Your information led me to the same conclusion.

The REAL solution is to reduce the function of the action so the pressure subsides in the case before te bolt begins unlocking. Go ahead and order MGI's adjustable gas tube. A little restriction of the gas system will make those Q3131 loads come out looking more like your lower pressure reloads.

So the addition of a Wolfe spring or MGI's 2X piano wire spring (talking buffer springs here) would not be a REAL solution because -- although they might work -- they are just masking the problem?

The brass on the corner of your bent extractor IS from it snapping over the rim. If you want to prove whether it is making the marks, just hand cycle a few rounds from the mag into and out of the chamber. If you are getting the marks during the extractor jumping the rim, you would get the same marks from just chambering and NOT firing. I'll pretty much guarantee you it's brass flow and over-function though.

Not getting marks from hand-chambering and the scoring pattern is consistent with bolt rotation direction on extraction.

Thanks.

I'm not sure that I know enough to manage an adjustable gas tube without blowing myself up.

Quoted: Adjusting the gas tube is EASY, you just turn a setscrew until the rifle runs right. Instructions should be included with it.

Does it have a "runs right" guage included?

Purchased and installed the MGI piano wire spring.  I haven't had a chance to shot it yet to see if it makes a difference.  

Since I'm wanting to re-contour the barrel anyway, I think I'll send it to Randall and have him check the gas port, etc., while he's got it.

What do you think, Randall?  Could I get you to install a YHM flip-up front sight while you're at it?