Dimples on Spent Cases, Dirty Chamber? - Page 2

olddav · November 14, 2014, 08:55 PM

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.

k Squared · November 15, 2014, 06:47 PM

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?

mag1911 · November 16, 2014, 06:25 AM

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.

Hummer70 · November 16, 2014, 08:09 AM

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-besid...est-enemy.html

It will give you lots of good information on how bad carbon is on your rifle.

wogpotter · November 16, 2014, 11:09 AM

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.

Dixie Gunsmithing · November 17, 2014, 10:41 AM

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

k Squared · November 17, 2014, 08:59 PM

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.

macsgunworks · January 18, 2015, 12:58 PM

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

November 14, 2014, 08:55 PM	#26
olddav Senior Member Join Date: December 21, 2008 Location: Lower Alabama Posts: 727	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. __________________ Never beat your head against the wall with out a helmet

November 15, 2014, 06:47 PM	#27
k Squared Senior Member Join Date: June 10, 2007 Location: Albuquerque, NM Posts: 238	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? __________________ K Squared

November 16, 2014, 08:09 AM	#29
Hummer70 Senior Member Join Date: June 22, 2009 Location: South Carolina Posts: 203	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-besid...est-enemy.html It will give you lots of good information on how bad carbon is on your rifle. __________________ Distinguished Rifleman High Power & Smallbore Prone President's Hundred (Rifle) US Palma Teams(2) US Dewar Team (2),4 Man Natl.Champ Team SB Prone Cert Test Dir. Sm Arms and Ammo,Aberdeen Pr Ground, Firefighter I, AC4HT Last edited by Hummer70; November 16, 2014 at 08:59 AM.

November 16, 2014, 11:09 AM	#30
wogpotter Senior Member Join Date: September 27, 2004 Posts: 4,811	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. __________________ Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”? Elderly Hunter: “That's dashed unsporting. Probably Belgium.”

November 17, 2014, 10:41 AM	#31
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	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 Last edited by Dixie Gunsmithing; November 17, 2014 at 10:48 AM.

November 16, 2014, 06:25 AM	#28
mag1911 Senior Member Join Date: March 3, 2011 Posts: 140	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.

November 17, 2014, 08:59 PM	#32
k Squared Senior Member Join Date: June 10, 2007 Location: Albuquerque, NM Posts: 238	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. __________________ K Squared Last edited by k Squared; November 17, 2014 at 10:46 PM. Reason: Correction

January 18, 2015, 12:58 PM	#33
macsgunworks Junior Member Join Date: January 17, 2015 Location: rhode island Posts: 5	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