New Build, persistent jamming

[Eye_guy]

I helped a buddy put together a Grendel. I’ve done a number of Grendel and 5.56 builds and haven’t ever had an issue quite like this before. The bullets (spent or live) will jam in the chamber to the point that the charging handle alone can’t remove them without excessive force.

We used a bcm stripped upper and an 18” Faxon fluted barrel. The tolerances on the bcm upper were such that we had to heat it with a torch (we did half a dozen rotations) to get the barrel on. (I’ve thermally fit other barrels and never had an issue.). After build completion, the rifle wouldn’t load a live round fully into the chamber. It would stop 1/2”-1/4” from being fully seated in the chamber.

After completing the build and having this issue, we swapped to a different bolt to no effect. Next step, we removed the barrel and put a second identical barrel on the same bcm stripped upper. Put the Grendel back together and still had the same issue despite a new barrel. So we stripped it back apart and this time swapped out the bcm upper receiver with a different brand and this time no thermal fit was required.

Despite a new upper and new barrel the issue persists. The issue wasn’t as pronounced as now the bullets will fully seat and it can be fired as essentially a single shot rifle but a live or spent round cannot be ejected even with the charging handle. Again, this issue persists even with a new barrel and a new stripped upper.

Thoughts?

[Klem]

Eye,

Sounds like the ammunition.

Either both chambers are too small, or the ammunition is too big - More likely the ammunition. Whether the build is a thermal fit or not should have nothing to do with it. GO/NO Gauges will solve this dilemma instantly. If you don't have ready access to gauges then a factory round is a sort of gauge. At least factory ammunition should be less than SAAMI.

What ammunition are you using? If you are using handloaded ammo then it introduces a variable. Just because your reloads work in your gun does not mean they'll work in anyone elses. Apologies if you know this already.

With the BCG out, can you drop an unfired round into the clean chamber with the barrel pointing at the ground. Press it with your finger to mimic the bolt pushing it into battery. Then turn the barrel up pointing at the sky and see if it falls back out. Doing this with factory ammo should have the round fall back out with gravity, or at least not stick.

[Eye_guy]

Klem,

Appreciate the response. My buddy is shooting hornady factory ammo.

And yes, with the bolt out a load can be pushed in and then it falls out.

All I shoot is handloads, but sounds like he needs to try some different ammo selections.

[JASmith]

The OP's first paragraph referenced "bullets" but it appears from the rest of the discussion that the OP is discussing the cartridge or a fired case.

Picking up on what Klem wrote, are you using factory ammunition in these functional checks? (Smarter than using reloads.)

Try coating the entire case with a dark magic marker and then chamber and eject the unfired case. There should be marks where the case and chamber fit is too tight.

I had this problem with my 20 Grendel wildcat. I found marks where the body and shoulder meet. Bumping the case a bit more during resizing solved my problem.

[Klem]

Eye,

So two barrels, two uppers, and two bolts all have compromised unlocking/ejecting. Unfired factory ammunition can go in and out of the chambers normally. Factory ammunition can also be fired, but won't eject properly.

I am now thinking it is a gas issue.

The common parts in all this are the carrier, the block and the gas tube. Maybe gas is restricted somewhere. If you turn off the gas, or fire an AR without a gas system expect to have to pry the charge handle with some effort, due to the case stuck in the chamber. Even a small amount of gas will unlock and unstick a spent case, and you can then manually work the action to eject it. But when there is no gas, cases become stuck in the chamber.

You could swap your BCG with his and see how that goes. It may be the gas key is loose but unlikely. It could also be something blocking the gas tube but again, unlikely and easily checked. That leaves the block.

Maybe the barrel's gas port is not lining up with the gas block hole. Are you pushing the block all the way to the barrel shoulder, or leaving the MILSPEC handguard gap (0.025")? Some gas blocks are machined to butt up against the barrel shoulder, and some only work with the MILSPEC gap With calipers you can measure the distance from the edge of the gas block to the centre of the hole, and then measure the barrel shoulder to the centre of the gas port and see if they are the same. Doesn't take much for the gas port and gasblock to be misaligned, and then you get what you are describing.

[just_john]

Adding a chapter 2 to Klem's, had a friend ( who is an armorer ) who built up a standard 556 and had similar issues. Did the bolt swap, barrel swap, go/no-go gauges, etc with no success. Turned out, the bolt carrier was chrome lined and the thickness of the plating was inconsistent where the bolt neck went thru the smaller aperature in the carrier and was briefly sticking in place. Replaced the carrier with a non-plated one and all was well.

[montana]

Turned out, the bolt carrier was chrome lined and the thickness of the plating was inconsistent where the bolt neck went thru the smaller aperature in the carrier and was briefly sticking in place. Replaced the carrier with a non-plated one and all was well.

All mil spec carriers are chrome plated on the inside..This is why many nitride carriers are out of spec because they are machined with the same specs as chrome lined and don't compensate for the chrome thickness..If the bolt was also chrome lined, then an excessive tight tolerance could occur..

The tolerance between the carrier I.D. boat tail bore and the bolt tail O.D. is .0019 to .0021..

The bullets (spent or live) will jam in the chamber to the point that the charging handle alone can’t remove them without excessive force.

If this occurred after charging a round, then there is more going on than just a gas issue..

Could be the barrels are getting clocked in the uppers (impeding bolt rotation and ejection), chamber problems, upper/ barrel extension tolerance issues, etc..

[Eye_guy]

Appreciate the responses.

I’ll swap out the entire BCG with one of mine from a functioning rifle.

We will try some different ammo as well.

Is it possible that the original stripped upper was out of spec and given the very tight thermal fit did something to both barrels that were mounted to it?

Can someone add some more color to this comment?

Could be the barrels are getting clocked in the uppers (impeding bolt rotation and ejection), chamber problems, upper/ barrel extension tolerance issues, etc..

[lazyengineer]

I helped a buddy put together a Grendel. I’ve done a number of Grendel and 5.56 builds and haven’t ever had an issue quite like this before. The bullets (spent or live) will jam in the chamber to the point that the charging handle alone can’t remove them without excessive force.

... Next step, we removed the barrel and put a second identical barrel on the same bcm stripped upper.

Thoughts?

My thought - it's your barrel throat. Replacing the barrel with the same barrel, means it still has the same throat.

For whatever reason, it's common for manufacturers to short throat a Grendel, so that mag length rounds with an agive that's forward of the case neck much any at all, will jam into the throat. Like - REALLY common; to the point of basically standard. 123 H's at mag-length are close on this already. Who's barrel are you using?

Try different ammo to see if they can seat and be withdrawn. Do the sharpie test (which isn't definitive). Maybe this is or isn't your problem - maybe it's just how this manufacturer cut the chamber. But with Grendel, it's so common to be short-throated, that on almost any issue be it reliability or accuracy, I pretty much auto-advise to always start looking there, at the throat depth.

[JASmith]

+1 for lazyengineer!

[montana]

My thought - it's your barrel throat. Replacing the barrel with the same barrel, means it still has the same throat.

For whatever reason, it's common for manufacturers to short throat a Grendel, so that mag length rounds with an agive that's forward of the case neck much any at all, will jam into the throat. Like - REALLY common; to the point of basically standard. 123 H's at mag-length are close on this already. Who's barrel are you using?

Try different ammo to see if they can seat and be withdrawn. Do the sharpie test (which isn't definitive). Maybe this is or isn't your problem - maybe it's just how this manufacturer cut the chamber. But with Grendel, it's so common to be short-throated, that on almost any issue be it reliability or accuracy, I pretty much auto-advise to always start looking there, at the throat depth.

I agree, but he stated :

And yes, with the bolt out a load can be pushed in and then it falls out.

If he did this correctly, then the bullet would have jammed in the throat preventing the cartridge from falling out..

[lazyengineer]

I agree, but he stated :

If he did this correctly, then the bullet would have jammed in the throat preventing the cartridge from falling out..

Maybe - but then, his own execution of that test appears to not mimic the degree of force the bolt is applying, since he experiences this issue even if unfired, if he lets the spring and bolt press it in. Could just be a crazy-rough chamber that's short on headspace as well, I suppose.

[Klem]

The idea a tight throat and maybe a rough or coated chamber doesn't sound plausible given he has tried two "identical" 18" BCM barrels with the same results. Plus as Montana pointed out, the test where you drop a live round into the chamber and reverse to see if it falls out freely would have it sticking.

I'm also thinking any interference fit between the bolt and carrier would be noticeable when they swapped and assembled both bolts into the same carrier. It still could be leaking gas at the BCG however if the tolerances were out, or the gas key loose. Not enough to cause a sticky spent case however.


Eye,
I don't imagine the tight thermal fit has anything to do with the problem. If you fitted the barrel all the way to where the barrel extension butts up against the receiver nose, and then cranked the barrel nut to its spec ft/lbs (which pulls the barrel extension in to where it is supposed to be) then all is well. You would have worked the action a few times when assembled and then fired the trigger. If the BCG goes into battery and the trigger can be fired then there's no reason the barrel/upper assembly will be the cause of a fired case jammed in the chamber.

[VASCAR2]

A Friend assembled an upper and the bolt/carrier wouldn’t go into battery. Traced the issue to the gas tube being to long. When installing the gas tube the guy didn’t get the gas tube seated properly. The retaining pin went in the gas block but ahead of the tube instead of through the hole in the tube. Makes me wonder if the gas block is not centered over the gas port which is affecting the gas or gas tube length or both.

[jasper2408]

Will be interesting to see what the problem turns out to be.

A couple of summers ago I was out shooting and a young fella showed up at the range and he had just bought a new Aero Precision, assembled from the factory, upper. The upper was brand new and had never been fired.

It would only shoot one shot like a single shot and would not eject the fired case. The bolt would not even move rearward when it was fired. It acted like it was not getting any gas. He had to force the case out by hand using a lot of force. It would load the cartridge from a magazine and go into battery just fine and it would fire the round just not eject it.

We did check the gas block and the tube and they were good to go and I asked him if he had installed anything new on the rifle. He said that he had put a new modular style trigger in the night before so I looked at it and found that the 2 tightening screws at the back of the housing had never been tightened or had come loose. I am guessing that they were never tightened and those screws stick up quite a bit above the trigger housing when they are most of the way screwed out. The trigger was pinned but was loose enough to stop the bcg from moving or those 2 screws sticking up were messing with it. We tightened the screws and everything started working correctly again. Shot real good after that.

Just my .02.

[Zeneffect]

Sounds like shooting steel ammo, then brass.

She a dirty girl?

[Bonas]

Start from scratch. Disassemble the upper. Clean the chambers thoroughly. Find one of the barrels in which factory rounds drop freely from the chamber. Reassemble with the looser fitting upper. Ensure the gas block is aligned properly. Blow through the tube to insure no obstructions. Test the BCG in another upper. In short, make sure each part works right before moving on to the next step. You'll find your culprit along the way.