Some time ago I purchased a Gunsmoke Enterprises AR pistol. Never shot it until 2 weeks ago when it consistantly jammed.
Haven't been able to fix it and here's the story.
1. Manually loading one round in the chamber w/o a clip, the gun fires and the brass usually remains in the upper damaged slightly by the bolt as it returns forward.
2. Loading one round in a magazine and firing the spent brass. Usually the case is left in the upper and the bolt may or may not stay open in the magazine detent position.
3. Loading 2 rounds in the magazine and firing, usually the fired brass is jammed forward along with the second round in the classic manner.
When I say usually, once in a while the gun cycles properly and ejects the round 5 feet out from the gun and will also ocassionally feed and fire another round correctly, but ultimately it always winds up jamming with a fired brass crushed in the receiver.
Here's what I've done so far.
Removed barrel and polished chamber. The brass doesn't have any frosted appearance.
Replaced extractor and spring with new parts.
Verified rings were 120 degrees apart.
Checked gas tube and block for operation.
Tried various ammo, Wolf, ss109, Remington, with various bullet weights. No change in symptoms.
This pistol has the short buffer tube, so I increased compression strength of spring slightly, and screwed the buffer cap in and out with no significant change in symptoms.
Examined brass under microscope. There is a very aggressive extractor witness mark on the rim of the fired brass, actually slightly deforming the brass and compressing the brass back about 15 percent of the thickness of the rim.
The rim is not otherwise distorted, just indicating that the extractor is pulling mighty hard on the case.
I haven't tried a new bolt because I don't have one available. Might buy one to try this WE.
I have a few thoughts what might be going on here but I'd like to hear some other opinions first.
I have a similar problem with an AR upper.
If the rim is deforming under the pull of the extractor and you have already polished the chamber my first guess would be that the chamber is slightly undersized or the ammo you are using is slightly oversized. The extractor is not strong enough to overcome the friction on the case in the chamber and is deforming it and slipping.
I would try a different brand of ammo first, maybe 2 or 3 different types. It might be the current ammo is just a bit too snug to feed properly.
AR pistols can be finicky beasts.
[ETA] sorry, didn't see that you had tried different ammo.
I've HEARD of some custom ARs having a problem were there is a "lip" at the entrance of the chamber. Not much. Can't even see it unless you disassemble the upper.
What happens is, the casing expands slightly when fired and takes up slightly more space then the exit.
Either way, it IS a friction problem and I'd look at either replacing with a more tapered chamber that will allow extraction or replace the upper completely.
Good luck, I know what it's like to have problems due to manufacture errors.
Thank you for your suggestions. I agree with your diagnosis.
I will purchase a new barrel to diagnose those areas.
Also, could you suggest a source of a carbide chamber reamer [I think my barrel is chromed], or is there some other way to properly size the chamber to get more clearance?
What spec chamber size gives best all around performance?
Could the gas block be allowing too much gas to the key and causing the extraction process to begin too early, before the chamber pressure drops and the case/chamber friction is reduced?
OK, I've found the problem causing the failure to extract, I think!
Tonight I took a dental pick with a right angle "hook" and probed the chamber.
To my surprise there was a groove maybe 80 degrees around the diameter of the chamber at the location where the neck of the case tapers to the larger diameter of the case, ie, just past the larger dimension of the "shoulder angle".
Then I looked at some fired brass under magnification and found what I had overlooked before, that the cases have a very subtle but distinct expanded band about .015 inch wide just past the ending of the neck taper [in the larger diameter direction].
A useful technique to see the band or other problems like this is to hold a straight edge across the case body lengthwise and using a magnifier and siloetting [spelling?] the contact of the straight edge and case against a bright background. Look carefully for any swelling or distortion.
Rotate the case and check it all around, if there is a problem you will see it as a distinct gap of light that profiles the fault.
So what is happening with my gun is that when it is fired, the pressure expands the brass into the groove, effectively locking the case in the chamber until the pressure drops.
BUT, the extractor grabs the rim of the case very aggressively and indents the rim about 15% of the rim's thickness leaving a very distinct witness mark. It slips off the rim as the bolt moves backwards.
As the chamber pressure drops, the case is blown out of the chamber and ends up in the upper receiver to be nicely jammed by the returning bolt.
That's why the case is not ejected out of the receiver.
No O-rings or any kind of super-duper extractor fixes will cure this kind of problem.
Anytime you have aggressive extractor markings on your brass, you should carefully examine your chamber.
This barrel from Gunsmoke Enterprises sure looks bad to me, and the most basic quality control, if they did any at all, should have picked this problem up.
I don't know, but I would guess that these barrels were rejects from the military or something, and were poorly chrome plated to fix a tolerence problem and dumped on suckers like me.
Once I get back to home base and look at a bunch more barrels and get this fully defined, I'll post a note on the main forum with detailed info for inspecting AR chambers.
If I can, I'll use a borescope with a right angle attachment to photograph the defect and post the pix.
Hopefully it might save others a lot of aggravation and maybe some $$$.
Great job on figuring this out!!!
I also thought of over-gassing the system, or extraction problems, but an f-ing grooved chamber is nuts!
Good luck with the fix, and I will now inspect all chambers carefully before I buy anything!
has gse been contacted and what do they have to say about it?
I would still try the Oring just in case. i have never run a AR with out one and never had any sort of problem.
I think you are over analyzing the problem. My pistol was doing the same and I just put an o ring around the extractor spring and it fixed the problem. Try the cheap easy aproch first.
I doubt the case was falling out on its own after the gas pressure in the barrel lowers. The extractor is extracting it out of the chamber just can't hold on to the case long enough to extract it out of the weapon.
As part of the troubleshooting after my earlier posts I did try the o-ring solution.
The extractor just dug into the brass more aggressively and I was concerned the extractor would eventially break off.
Don't misunderstand, I think the O-ring has it's purpose and I'll probably use one all the time.
But we're talking a deformation ring on the brass caused by a groove in the chamber that can be seen just by probing with a dental tool.
I personally wouldn't call that over-analyzing. But I'm often accused of it!
I'm just pointing out reality.
If I hadn't stuck with trying to solve the problem like many folks I personally talked to suggested, where would I be now?
They wanted me to send the gun to a gunsmith, back to the factory, etc, etc.
So I didn't and stuck w/ it and now we all can share the knowledge gained. I know I've learned a heck of a lot...
Your next point, that the case is still being "extracted" and that you doubt that it is being blown out.
Well, if that's the case, then why is the case not being ejected out of the ejection opening and instead ending up laying in the chamber or being squished when firing one round from a mag?
If your pistol was doing the same as mine and the brass had a deformation ring and I bought it from you, then I'd sure do a little analysis to find out why I needed an O-ring to make it run.
RR, your AR operation sequence flow explanation is excellent, as all have said! Thanks!
Sounds like an overfunction (i.e. action moving too fast) issue, like others have said. Mine did the same thing. Use a heavier buffer to slow the action down and I bet your problem dissapears.
I had a similar issue with my AR pistol. I got the upper and all the parts minus the receiver from Double Star. I did the chamber polishing but I have a chromed bore so I am not sure what good that did. I continued to have double feeds and FTE's. DS sent me an improved extractor spring and ring. My pistol works perfectly ow. I can see the place where the extractor pulls the rim. I can not see any abnormal rings or marks on my brass otherwise.
Thank you sir. I had not seen that post. I do have some questions generated by your post. I have a full CAR buffer spring in a short Pistol buffer tube. The buffer does not seem to be impacting the back of the buffer tube. Recoil is on par with my other AR's. I will post a pic of the case head ASAP. Do you still suggest I have an overpressure during extraction issue and if so how would I best fix this?