Bad day at the range ! I just finished a 300AAC Blackout 9" AR build. Went to sight in an EOTech and play around a little bit. I got through around 10 rounds of reloaded 150gr FMJ's (my own reloads) then my bolt would not go fully into battery. I cleared the rifle and inspected everything. Tried again with a fresh factory bullet and same thing. After I got home, I cleaned the entire rifle and tried to seat a dummy round. Still couldn't get the bolt to lock. With a little persuasion, I was able to get the bolt to lock up but then it was stuck! I had to beat the charging handle with a wooden block to get it to cycle back. The bullet slid right out so it wasn't the casing that got stuck. This leads me to believe that I have a headspace issue. I'm no gunsmith but I believe that I have negative headspace??? Does that make sense? I don't think that the bullet is being allowed to sit deep enough in the chamber which is preventing the bolt to go into battery. Any idea what could cause this? Could it be a reloading issue?

These loads were relatively tame. 14gr of LilGun on a 150gr FMJ 2.065 OAL. They usually run at around 1750 fps.

Did you try any factory ammo?

Well... yes; it could be a reloading issue... But.

If your OAL is too long the projectile could be jammed into the rifling. that usually gets the projectile lodged into the throat or even as for as the rifling. When this happens to projectile is usually pushed into the case but even with no crimp there will be marks on the projectiles ojive (from the barrels rifling) these will be slight scuff looking marks. If this is a sever case the bullet will be lodged in the bore when the case is extracted.

If you case neck is too long it will be jammed forward and the end of the chamber will be used to over crimp the projectile into place (and jam that thing in but tight). there will be a bright line around the case mouth (usually). The projectile will not be jammed into the case but the case over crimped on the projectile the whole damned thing jammed and stuck in the chamber.

Same symptoms for a chamber that is way too short.

Need to check ammo; if that checks out in a drop-in chamber gauge; YOU HAD BETTER CHECK THAT HEADSPACE

Yes I have tried factory ammo...same problem, can't get the bolt to lock. I took a correctly resized and trimmed case (no bullet) and put it into the chamber and eased the bolt forward. Still would not go into battery. So from that I am fairly sure that it is not a bullet sizing issue. My only guess is that the headspace is negative. Which doesn't make sense! Why would it function flawlessly for 10 rounds and then all of the sudden the headspace is negative.

Some sort of debris got left in the chamber?



 

I cleaned the damn thing as soon as I got home...still having issues lol

Time to buy some guages.









 

You have checked the operation of the bolt, right?  Pulled it apart and made sure it functions as well as inspected the barrel extension?

Yea, it is AAC's 300 BO barrel. I'm going to get some gauges on Monday.

Yea, I've swapped out bolt carrier groups as well. Barrel extension looks clean. May need to take the barrel off to get a better look though.

Not a headspace problem, as it fired the first ten rounds before you had the problem and headspace increases not decreases. Headspace also ruled out as you have tried a different BCG with the same results. I would bet on debris. Did you keep/check your spent brass? Does the bolt go into battery on an empty chamber? I think you had a popped primer and you will find it securely wedged between the top of your gas tube and the inside of your upper receiver,  which is impinging your gas key and keeping the bolt from going into battery. This would explain why both BCGs would not go into battery and why when you forced it nearly into battery the one time you needed to hammer the charging handle with a block of wood to pull the bolt back.

It's not debris related...after cleaning the rifle, it is still having issues. I did find my casings, they look normal. No signs of wear or excessive pressure. The bolt will freely go into battery with no casing in the chamber. Anytime there is a casing in the chamber it prevents the bolt from locking up. Factory rounds as well.

Does the bolt close on an empty chamber? Debris such as a popped primer could be wedged between the receiver and the top of the gas tube without you being able to see it.

Don't see how it could be anything other than debris (except a misaligned or deformed gas tube) as head space is set with the installation of the barrel extension, The only thing that could change headspace in the span of ten rounds would be the barrel extension coming loose. That would be pretty hard to happen as the indexing pin would stop it from rotating, as well as would the barrel nut pressure on the shoulder of the extension. If the extension did manage to loosen, it would increase the head space, not decrease it. the only other thing that it could be is some sort of deformation or burr on the back side of the bolt locking surfaces of the barrel extension, which is also highly unlikely.

If there really is no debris and the gas tube is aligned and not deformed then it may be a headspace issue if the chamber was not finish reamed. This is highly unlikely though, as you had ten rounds function normally before the problem manifested. A manufacturing defect in the geometry of the BCG can most likely be ruled out as well, as the same problem was exhibited with another BCG, which means it's not a loose gas key either. You could try different factory ammo, but I doubt that would solve anything as you said a dummy round would not allow the bolt into battery either.

So, still think it's debris or possibly misaligned or deformed gas tube.

The bolt closes and locks up freely with no round in the chamber so I don't think it's debris. What you said about the headspace changing makes sense, I don't see how the headspace would decrease any over time. This is also not a cheapo barrel so I would think it would be rare to be a manufacturing error. A member of another forum suggested that maybe I simply need to break in the rifle. He said that the headspace may be tight and a little grease on the locking lugs and barrel extension lugs may help. He was having similar issues with a 6.8SPC

Well, if you are sure that it is clear of debris and the gas tube is properly aligned and not damaged, then the only other thing I can think of is that maybe the upper receiver notch for the barrel index pin is slightly enlarged and the barrel may have slightly rotated, causing a timing issue between the barrel extension and bolt, but that should cause some resistance when closing the bolt on an empty chamber as well. If you had not been able to run the first ten rounds without issue I would suspect an improperly reamed chamber and a headspace issue. Only options I can suggest at this point are to reinstall the barrel and gas tube, square the receiver and reinstall the barrel and gas tube, start replacing parts in increasing order of cost (gas tube, receiver, barrel). Good luck.

The BE is only one of the factors that determine headspace.
If your problem is not headspace it should be plainly visible with a good inspection.

The gunsmith that assembled my upper told me that I needed to ream my chamber. I'm sorry but after spending $425 on an AAC barrel, I shouldn't have to do anything to it!

Went to another gunsmith here in town who seemed much more knowledgeable and took the time to inspect the problem, is fairly certain that the barrel I received is out of specs. When the bolt is forced into battery on a round, the barrel leaves a ring right below the shoulder indicating that the chamber is a hair to short to fully chamber the round.

Once I can get a hold of AAC I will be requesting a new barrel, we will see what they say!

I have a similar Issue with a 5.56 Oly barrel . They claim because their barrels are made to tighter tolerances they head space the barrel to the bolt . So i picked up second hand oly barrel and 3 new bolts and none of them will close on a GO gauge and and it will not chamber a round.

They said if i have a Oly bolt and barrel  they will head space it for $20 plus shipping. I don't have a oly bolt and I sure wont be buying any more Oly products especially if they don't even make their parts in spec with their own parts

If the rifle will not function and fire with factory ammo I would say you  have an issue...........Try swapping out BCG and see if the problems cease..........If that does nothing for you........then get with who ever made the barrel...........I have built two 300 BLKs..........and test fired the first build with factory Remington Supers..........tweaked my reloads and went with the second build..........

Exactly my thoughts! You should not have to pay extra money to have the chamber made right lol! And if you have to adjust headspace on an AR you are having serious issues.

It's not about not firing, I'm sure it would fire just fine. The rounds (even factory ammo) will not seat in the chamber far enough...It's not the BCG it's an out of spec chamber, I'm 100% sure about that.

AAC paid to have the upper send in and they are going to replace my barrel for free! Can't wait to get it back!