This rifle used to eat everything flawlessly. It sat in the safe for the better part of 2yrs until I got back with it recently. I've been experiencing this failure to go into battery with every load put through the gun, reloads and military ball.

I've throughly cleaned and properly lubed everything and tried new buffer springs and even one of the JP silent captured units. When it does cycle it will lock the bolt back after the last round so I don't believe I have a lack of gas issue but I'm willing to listen if you can point out why that might be a concern.



The cartridge enters the chamber about 80% or so and hangs up. The bolt is very difficult to retract when in this condition. When the bolt is racked the cartridge will freely enter the chamber. I've experienced this with multiple mags.

Any suggestions?

CR

Have you pulled the bolt from the carrier to look at the rings? Does the bolt move easily in the carrier.

Have you thoroughly cleaned your chamber area with solvent?

Sounds like gunk has built up over time while it sat.

is the chamber rusted? That sounds like the problem.

You said it used to be reliable with several ammo types previously so it isn't likely a major fix.

If the springs are new, the rifle is truly clean and lubed appropriately then I too would check the chamber. I too am in the "run them wet" group.


Pop the upper open and with a flashlight make sure the chamber is truly clean, I'm betting lube and carbon have it gummed up. If it is truly clean try to drop a round into the chamber. It should drop right in.

Next strip the bolt and make sure it is clean of fouling and old lube, scrape the carbon off the bolt tail, check that the gas rings still hold the bolt up and that the gaps are offset. Lube lightly and reassemble.

If those don't fix the issue I would pull the gas tube and thoroughly clean it with solvent. Gas AR's are very picky about port size and tubes being dirty. Its rare but all AR's eventually need the gas tube cleaned.


My guess is the issue is one of those first 2 things most likely due to old lube and grime gumming it up.

Everything is spotless including the chamber. It's hard to tell in the picture but the head of the cartridge is being pushed up, sometimes hard enough to dent the side of the case. When you retract the bolt a little the case will drop right into the chamber.

I'm going to pull the gas tube and block off the barrel today and verify that they're clear. What makes me think it's not gas related is that I can simulate the failure with dummy rounds while the rifle is in my vise.

CR

What about your ejector? Is it protruding and catching on the case rim?

I'm assuming you mean denting the top side of the case as it enters the chamber.

Are the feedramps lined up?

I can't imagine the extension pin shearing, but sounds like something has to pushing the base of the case up and binding the BCG.

Ejector and extractor are free moving and appear to be working properly.

Yes, dents on the top side of the case.

Barrel extension hasn't moved as the M4 style cuts are in the right spot.

To me, it's acting like the bolt is moving too fast, not allowing the case to smoothly transition into the chamber.

CR

Have you tried more than one magazine?

Like the bolt is slamming the case towards the chamber as the round is popping upward and free of the magazine feed lips.

Could the magazine catch be holding the magazine too low? High enough for the bolt to strip a round, but lower than the optimal angle for feeding?

I have tried multiple magazines with the same results.

Generally gas gun woes aren't that difficult to cure but this one has me stumped.

CR

Tore the bolt down completely this morning and found the ejector retaining pin looking like this. I believe it was allowing the ejector to protrude out the bolt face too far.





Can I assume that pin isn't supposed to have that groove in it?

CR

Replaced the ejector spring and pin and that cured the feeding problem.

She ran flawless this morning.

CR

Glad you got it fixed. Thanks for the update.

Thanks for posting. Informative thread.

A member over on Snipers Hide offered this explanation of what he thought was happening.

"If the ejector protrudes out of the bolt face too far, it will deflect the case sideways toward the ejection port just like you see in the pic. As the case feeds up from the bottom, it encounters the SIDE of the ejector instead of the END of the ejector. This causes the case to deflect sideways just enough to cause it to bind as the rear of the case will be trying to enter the chamber at an odd angle when the bolt comes up behind it. Since the case is against the SIDE of the ejector, it cannot correct itself fast enough before it binds."

Seems to be pretty much spot on.

CR