Dimples on Spent Cases, Dirty Chamber?

I expected to see more than that on the patch from powder not completely burned and case not completely expanded when the bullet left the barrel. As mentioned you normally see soot on the case if you don't get full case expansion. Might try shooting once then extract the second round. Feel for debris on the second round, look up the chamber/try the patch again.

You might try cleaning the upper/lower assemblies and magazine completely with something like Gun Scrubber. Possibly something carried in by the other rounds after being shaken loose by recoil.

If that doesn't work I don't know what to tell you. These guys might have some ideas:

http://www.ar15.com/
 
No Dimples With Factory 5.56

Long delay in getting back to the range, but today I fired some 5.56 factory ammo and the dimpling was significantly reduced.
I started by cleaning the chamber then fired 10 rounds. The picture shows (from left to right) the first, second, third, and tenth round. The picture has a reflection on a couple of the cases, but there is almost no dimpling. I did NOT clean the chamber between firings.
Dimple-556J_zpsef50fc78.jpg

The length of the first set of fired cases (with dimples) is within a couple thousandths of a inch of the second set (no dimples). I'll have to get my head space gauges out of storage to check the difference between the factory ammo and my reloads.
Could the reloads have been sized too small for this chamber?
 
I wouldn't think so but I'm not an AR shooter. Two quick questions, the black marks on the brass is it a reflection, and what charge are you using for your reloads?
 
Charge

The reloads were 55gr FMJ BT with 26.7gr BLC -2 or 22.3gr H335.
I use CCI military primers. I sized the cases with a RCBS small base die, with the die screwed down to give a slight "cam over".
I guess it could be a case of all the tolerances going the wrong way.
 
I don't think it is from the brass being stretched. When the cartridge is fired, the pressure from the charge is pushing outward, against the chamber walls, which would not allow dimples to form inward, I would think. What the brass might do as it cools, though, I do not know. I have never seen this before.

If it were a headspace problem, you would see bulging and head separation, etc.
 
I'm thinking (scary) that what ever it was is gone now, if when you shoot it again the brass comes out fine I'd say problem solved or at least disappeared.
 
Further Trouble Shooting

Today I made another trip to the range a tried removing the gas tube and firing the reloads that had turned up with dimples.
The bolt carrier stayed locked after firing, and I manually cycled the charging handle to eject the spent case and load the next round.
I fired 10 rounds and had NO dimpling at all.
I'm guessing that gas & unburned powder were entering the chamber from the back end as the bolt cycled. If this is in-fact the problem, I've got no idea how to fix it.
Any suggestions from the experts?
 
I'm certainly no expert but it has to be something blown into the chamber after ejection of your reloads. If your rifle and magazine are completely clean I can't think of anything other than unburned powder causing this. Have you looked for unburned powder in the barrel?

A little dimpling on the 10th round with factory ammunition might indicate an issue with the mechanicals and/or tolerances in this particular rifle.

If I were doing this for myself I would try:

Reducing the powder charge to the minimum load to see if I got a more complete burn.

A different lot of powder(s) but if this happens with two different powders already I don't see this fixing the problem.

A different lot of primers or a different manufacturer.

Next I would try an IMR style powder.

Good luck.
 
I have seen that and worse. I believe it is rust. ARs had major problems with rusted chambers in Nam in the first issue rifles. To fix this they chromed the chambers first then chromed the bores.

Brass left loaded in a chamber in damp climates sets up a condition known as galvanic action (rust).

Worst case I have seen was a buddy went hunting in the rain, came home and wiped outside of rifle off and put it in cabinet and left it loaded. Went out to shoot it again and had trouble opening the bolt after firing.

He brought it down and I borescoped it, chamber was heavily rusted. I cut a couple threads off and rechambered his barrel and that removed the pitting in body area but he still had some in neck.

I also recrowned his barrel and he went out and shot it again and reported it had never shot so well.

You might put some emery cloth on a cleaning rod or wrap some around a brush and insert in electric drill and polishthe chamber as with a AR you won't be able to set the barrel back and rechamber it.

The black residue you got from inside case mouth is propellant residue and is carbon. I did a search and apparently I did not put the below info on this forum so to save lots of typing check out:

http://gunhub.com/rifles/48303-besides-politicians-our-biggest-enemy.html

It will give you lots of good information on how bad carbon is on your rifle.
 
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Guessing as I'm not there hands on, but it sounds like incomplete powder combustion before reaching the gas port.
Maybe a different powder, magnum primer or something else to get a better combustion rate.
Drastic action would bee convert to a gas piston system instead of DI.
 
There shouldn't be any way for gas to get in the chamber from the bolt end. The gas comes into the port of the bolt key on top the bolt, via the gas tube, and runs down into the bolt carrier, to drive the rear of the bolt carrier back, which causes the front of the bolt to turn and unlock. This should take place before the gas tube comes out of the top of the bolt or the "key". The only other path would be around the firing pin, but the gas should just go out the bolt, instead of doing this, over the way its designed, and being as the gas would hit the rear of the cartridge, if it did come out the firing pins hole on the breech face, it shouldn't make it into the chamber. Also, there is a set of gas rings in the bolt to stop forward escape of gas. You might try changing those, and maybe the bolt key, but I highly doubt this being it.

AR15 M16 Operation And Functioning Cycle, Start at 10:00:

https://www.youtube.com/watch?v=NtwhZj1_TlI
 
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You guys were right

Mag1911, olddav, and wogpotter,
You guys were spot on. The problem was unburned powder getting into the chamber. The unburned powder may have spilled out of the fired case as it was extracted.
I changed to a faster burning powder (Hodgdon H4198), and the problem cleared up completely.
I just took a long time for me to accept the fact that a load that worked well in one gun would preform so differently in another gun of the same caliber.
 
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think out side the box

is this the first time you fired this gun. check the gas tube and gas block alinement .make sure gas block hole is lining up to barrle perfect and then check the gas tube . this looks like over pressure. the gun will cycle without gas tube workin with litte weight buffer spring . just that little extra pressure let out threw the gas block activating the bolt rearwards a few hundreths of a second faster prevents the over pressure it looks like you may be getting. i would look there first. good luck
 
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