Dimples on Spent Cases, Dirty Chamber?

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I tested fired a new AR upper and the cases came out with dimples.
SpentCases_zps4e90d05f.jpg

Would this have been caused by a dirty chamber?
I cleaned the barrel thoroughly, but just whipped out the chamber.
 
That looks exactly like something I aint never seen before. Brings to mind an article in the 1979 Speer Reloading manual. But, those dimples were severe, quite long, and confined to one side. I believe they were contributed to too light of a load. I don't see this as being the case I don't think.

What ammo are they? Reloads? Remanufactured?
 
These are reloads, but...

These are reloads, but I've fired some of the same batch from another AR without this problem.
 
It could be from dirt, or from too much oil in the chamber
http://www.saami.org/glossary/display.cfm?letter=O
OIL-DENT
A defect in a brass cartridge case, usually in the form of a shallow indentation caused by excessive oil present in dies during case manufacture, or in re-sizing dies during handloading operations, or in a chamber during firing. Also called fluting.
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I'd polish the chamber with some Flitz, then clean and LIGHTLY relube
 
The oil dent theory is possible, but I think it would have shown up when I fired some of the same ammo from another gun.
I'll start scrubbing and report back.
 
The oil dent theory is possible, but I think it would have shown up when I fired some of the same ammo from another gun.
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Not if the excess oil is in the gun
It's very unlikely it's on the cases after being handled

I'm betting a good cleaning is going to fix things
 
Unburnt powder granules give a similar effect. Its "sharper edged" than oil dimples & each dimple is too small. I'm guessing either powder, or failing that machining swarf particles from manufacture.:o

Did you have some kind of misfire/squib or something else unusual before this happened?

Are there unburnt grains scattered elsewhere in the action?

Give the chamber a thorough cleaning.
 
Additional Testing - Dimpled Cases

I ran the following test to help trouble shoot this problem.
I thoroughly cleaned the chamber (and bore).
I fired 3 shots.
First case (on the left) has no dimples (reflections in photo appear as dimples, but are not)
Second case (middle) has significant dimpling.
Third case (right) has dimpling similar to the second case.
I repeated this test three times and got exactly the same results each time.
I had of mix of military & commercial cases (reloads that did not dimple in other ARs).
I assume the chamber is oversized for these cases, which is allowing powder to blow back along the side of the cases.
Any other theories?
IMG_13631_zpsd2ce3527.jpg
 
I don't think it's gases. It's more like the brass hit the ground hard and had pressure applied to them.

In a clean chamber, the brass is more likely to stretch.
 
Brass Catcher

I used a brass catcher. The cases never touched the ground.
I also noticed the dimples all show up on the bottom (6 O'clock) position of the chamber.
 
Clean the chamber again. Fire one shot and look in the chamber. That much debris should be easy to spot if it's powder granules. Maybe run a loose swab up there with just enough oil on it to catch anything too small to see.
 
Swab After One Firing

Mag1911,
Per your suggestion.
IMG_13491_zpscd3bf6a3.jpg

The black ring on the swab was darker than it appears in the picture.
Is it possible that the fouling is being caused buy the case being ejected before all the powder is burned/leaves the case?
I have used two different bolt carrier groups, and gotten the same result.
Also, the gun cycles without any problem, cleanly ejecting the spent case and picking up and chambering the next round.
 
Cartridges that I have seen, which have had a gas leak occur, had a black streak, similar to soot, on them. If they don't have that, then I would be looking for something else. A bore light might help, but casting the chamber, after it is thoroughly clean, would be the best bet. You can measure it for concentricity, correct diameters, etc., along with seeing any imperfections.

I need to add, that it was an AR that I saw this happen with, over at the range one day. I didn't notice until the shooter was gone, and happened to see some of his brass laying around.
 
I assume you are not using ball powder, but what powder are you using. If you cleaned the chamber and the brass from first shot was clean but all subsequent brass has dimples that sounds like unburnt powder to me. As a disclaimer, I'm not that smart and may be incorrect.
 
If you cleaned the chamber and the brass from first shot was clean but all subsequent brass has dimples that sounds like unburnt powder to me.
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That sounds right to me also.
There's really nothing else it could be if the first case didn't have dents
 
Yes, I'm using ball powder, but...

I have been using Hodgdon BLC-2 and H335 (ball), with CCI military primers, but I've fired the same loads from 2 other guns without any problem.
Casting the chamber is no doubt the next step in trouble shooting, but this is not an expensive barrel, so I may just replace it.
 
That looks exactly like something I aint never seen before.
Click to expand...
Okay, I've read this four times and I think it's funnier each and every time I read it but I still can't be sure what you meant when you wrote it. Unless it was simply meant to be funny, in which case: nice! :D
 
Well I no longer think is unburied powder. Perhaps something coming from the bolt as it comes forward? Something is depositing after the first shot but you already know that.
 
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