AR15 Headspace Issue

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ingEneer22

New member
Bad day at the range :mad:! 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?
 
Yup, I'll be doing that Monday. I'm more curious than anything to know what happened to cause it to do this. I'm willing to bet the barrel is out of specs.
 
Not likely the barrel. Probably your bolt. We work on a lot of AR pistols, and it amazes me that people always think that the problem is the barrel. :rolleyes:
 
Will the bolt lock with no round in the chamber? Have you tried the plunk test? Have you checked for something like a loose primer stuck behind the locking lugs? Can you manually seat the bolt by hand with the upper separated from the lower?
 
The bolt locks fine with no round in chamber...it's not a debris problem, I've cleaned the rifle and still the same problem. The only time it has issues is when a round is in the chamber. I've tried factory ammo too.

What is the plunk test?
 
What is the plunk test?
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That's when you hold the barrel vertically and drop a round into the chamber to see if it makes a solid "plunk" noise when the shoulder of the case hits the shoulder of the chamber. And then reverse the barrel and see if the round falls out on its own. It's just a cartridge/chamber fit test.

Do you have any spent brass from the first 10 rounds to see if it will chamber?

It just sunk in that it is your own build. Did you set the headspace with a set of go/no-go gauges? If not, how? If so, have you rechecked it?
 
That's when you hold the barrel vertically and drop a round into the chamber to see if it makes a solid "plunk" noise when the shoulder of the case hits the shoulder of the chamber. And then reverse the barrel and see if the round falls out on its own. It's just a cartridge/chamber fit test.

Do you have any spent brass from the first 10 rounds to see if it will chamber?

It just sunk in that it is your own build. Did you set the headspace with a set of go/no-go gauges? If not, how? If so, have you rechecked it?
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A round will drop into the chamber, and freely fall out when turned upside down. I had a qunsmith assemble my upper. I'll take it back to him on Monday to get the headpsace checked.
 
One last SWAG and I'm done. Have you checked the ejector to make sure it recedes into the bolt face when you press on it? If it works okay, then I say the first poster who mentioned headspace wins a prize.
 
I worked on thousands of M-16 and A-1s. We can only guess, but the symptons sound like your gas tube (Key) on the bolt has a knick in it. The area around the front is quite soft and knicks easily. I would have said the gas tube was upside down, but you said you had already fired it. You could run a carrier in without a key on, or remove the gas tube, and then you would know if it was in this area where the problem was coming from. Maybe you forgot to pin the gas tube?
 
Do you have a Go headspace gauge, to try in place of a cartridge? That should be the first thing a Gunsmith will do. If the bolt will close on a Go gauge, then it should close on factory new ammo.
 
I worked on thousands of M-16 and A-1s. We can only guess, but the symptons sound like your gas tube (Key) on the bolt has a knick in it. The area around the front is quite soft and knicks easily. I would have said the gas tube was upside down, but you said you had already fired it. You could run a carrier in without a key on, or remove the gas tube, and then you would know if it was in this area where the problem was coming from. Maybe you forgot to pin the gas tube?
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If the gas tube or gas key problem would prevent the bolt from locking on an empty chamber correct? The bolt is not going into battery ONLY when there is a round or casing in the chamber. The bolt operates freely on an empty chamber.
 
Do you have a Go headspace gauge, to try in place of a cartridge? That should be the first thing a Gunsmith will do. If the bolt will close on a Go gauge, then it should close on factory new ammo.
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I don't have a GO gauge, but I will be taking it to a gunsmith tomorrow. I did lightly grease the locking lugs on the bolt and the barrel extension and that did help a bit. Another user suggested that lightly grease those surfaces and put a few more rounds through it. He though that maybe it needed to be broken in a bit.

I will be taking it to the gunsmith though before I put anymore rounds down range, just in case the headspace is off.
 
One thing to check, is that the ejector will push in to at least flush or below the bolt face. If it sticks out, it could hold it off from closing.
 
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!
 
Do you know anyone you could borrow a chamber reamer from? A finishing reamer would be ideal, and a tap wrench, along with some cutting oil.

If they don't take the barrel back, that would be your next alternative, to save some dough. If not, then you'll need to find a gunsmith who knows how to finish chamber one. It sounds like it only requires very little reaming.
 
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