I've owned and shot AR15s for 30 years now, I have 4 uppers, .223, .450B and 6ARC, (the newest build and the subject of this post). I built this upper a few weeks ago and I'm 150 rounds into it. No feeding issues at all. Today I'm 18 rounds into a 30 round 1/4 mile match, (which I am winning handily), and I get three jambs in a row. Nothing that I have ever seen before in any chambering. Has anyone ever seen anything like this? The cartridge is jammed 1/2" out of battery and looks like it was wedged into the chamber at an angle? The first two I ws feeding from a Bobsled, figuring that was the issue, (it was a .223 bobsled, not specific to the 6ARC), I loaded 10 in a duramag and the second round did the same thing. I pulled the bolt between each jamb and did not see any obvious issue. The only change I made prior was to put in a new recoil spring the day before the match, and I shot 40 rounds with the new spring before these jambs. Look at the pics and help me out here if you've ever seen this before
If the neck is hanging up on the mag forward edge, this is a common issue for the 6 Arc. Use a dremel and lower the front edge about... 0.12" or so, there's a thread on this. Mark it with a pencil or something first, so you don't dremel too far down. Leave any spot welds on the front facing side in place, I've done this for all my 6 Arc mags.
I should have mentioned in the original post that I was single loading these using a Bobsled during the match. After the first two jams I tried loading from the magazine with the same result. So it can't be a gas issue, and it doesn't seem like it could be a magazine issue. I'm going to pull it down this afternoon to have a good look at the lugs and chamber and also pull the new spring and try the old spring again to see if that makes a difference.
While you have it apart put some of that blue metal working die on the feed ramps and run some rounds through it. Would be interesting to see what path they take off the sled. Put some on the front of the magazine as well.
If you have not solved the problem all ready, a few things to check. What does a bore scope of the chamber show?
Also Factory ammo or reloads? If reloads measure the case o.d. dimensions at the neck and shoulders of the loaded rounds to see if they are too large. It appears that the rounds are being squished because the front of the case goes part way into the chamber and can't go any farther, possibly a misalignment issue.
06-07-2024, 04:28 PM (This post was last modified: 06-07-2024, 04:30 PM by BD1.)
So, I replaced the new spring with the old spring, and the rifle runs again. However, I also took a close look at the chamber, throat and barrel with a fiber optic inspection camera. All look good, but as I was doing that I noticed that the ridge in between the M4 feed ramps of the barrel extension comes to a sharp point. That point has not been beveled as is the case in my other ARs with M4 ramps. That explains the little divots in the shoulder. My current hypothesis is that the new recoil spring added just enough extra speed to the bolt carrier that it was spiking the shoulder of the cartridge on that sharp point between the feed ramps causing the cartridge to tip down on the way into the chamber leading to the bend, and the jam. I had intended to pull the barrel and bevel that point and polish up the ramps this week end, but I am out of polishing tips for the Dremel and no-one in town has any. So I ordered some online and I'm shooting it the way it his until they get here and I have a day to tear it all down to do the work. I'll get some pics when I have the barrel out.
So I pulled it apart and beveled the point where the M4 feed ramps come together and polished the edges of that ridge and both sides of the feed ramps and all is good. I'll try and post a pic or two when I get some time.
30 rounds today with no issues.
So the mystery continues. I pulled the rifle apart, beveled the point between the feed ramps, polished all of the leading edges around the feed ramp and put it back together. it ran perfectly for 50 rounds of load work up, netting me a load for the Berger 109s that worked. So today I showed up for the next 1/4 mile match, full of confidence, and on the fourth round it jammed again. WTF again? I have 200+ rounds through this gun at this point and this jamb only happens during a match? I tried easing in a round and closing the bolt using the forward assist and two shots later it jammed again. So I was frustrated, but got going, single loading from the Bobsled and finished the first string. The jammed brass looks the same as last time, so I'm thinking it has to be hitting the back of one of the barrel extension lugs at the top of the chamber. Between strings I took a marker and put a line on every cartridge. I shot the next two strings by standing up and loading each cartridge while carefully turning the line to the top so I could tell which lug was causing the issue. Of course they all fed perfectly after that. Problem being somewhere along the way messing around with getting the cartridges aligned I miscounted a string and only fired 9 shots, (instead of 10). I'd like to blame my second place finish on that alone, but I was down 14 points in the final score so even if the shot I dropped was in the 10 ring, it wouldn't have quite made the difference. So now I guess I'm pulling it apart one more time to look closely at the back side of the rest of those lugs. Although I can probably polish those without removing the barrel as I don't have to worry about buggering up the aluminum ramps in the lower this time. The bright side is that I stumbled onto a load with the 105 grain Bergers after the match. So the day was not all bad.
