Ok I recently built this ar pistol, it's a MIT of sorts, Para mil spec lower with bushmaster parts kit, ar pistol buffer tube with spring and buffer ( midway USA ). I bought the upper assembly assembled, the usual tricks 6" barrel yankee hill low pro gas block free float hand guard. Now for my issues I took it to the range and I was attempting to shoot PMC .223 brass out of a Magpul p mag. I loaded the mag to capacity right out of the gate the first shot it didn't eject the first round before attempting to load the 2nd, I disassembled the upper, found the gas rings on the bolt to not be properly spaced, I oiled everything spaced the rings and re- assembled. After some more tinkering and firing long story short I couldn't get it to feed more than 3 rounds with the same issue, failure to eject. I forgot to mentiion the upper has been headspaced and test fired prior to my firing. So what's my issue is the the ammo? I plan to hit the range with it again with some federal 5.56. Any info would be greatly appreciated!

I suspect a weak extractor, I do not think it is the ammo

I was wondering about the extractor but being a new build I was hesitant, the bolt carrier group came with the upper so it's prob cheap....ugh, what would you rec. doing to remady my problem, get a new bolt? New extractor spring?

Short answer, add a #60 o-ring. To the extractor. Around the spring. Sounds like your problem. Test firing usually is only one round. Last one I got the round still in the chamber.

Thanks for the reply! Yea my guess was that it was only one round.... I'll give the o ring a try.

Is it failing to extract, or failing to eject? What I mean is, is it failing to pull the spent case out of the chamber, or failing to get the spent case out of the port?

this thread has a good discussion of the issue(s) you are probably having

Load 1 round in the mag. Fire it. If the bolt locks back on the empty mag, it isn't short stroking. Do this a few times to make sure. If it isn't locking back, try a standard carbine buffer if you are using an H or heavier.

Make sure your gas block is aligned correctly with the gas port. You can cut gas flow down by not having it centered.

I always use a BCM extractor spring upgrade. Helps insure extraction.

The other thing is always use full powered ammo for break in. Is it a .223 chamber or 5.56?

Clean and lube it well again before returning to the range. Scrub the chamber.

I also lube and hand cycle my new builds 200 times before ever going to the range.
Dave N

Dave, thanks so much.... I am going to disassemble and clean tomorrow and look everything over, to answer the other question it is failing to eject... If you load only one round n the mag it will fire and the bolt will catch, so as said before it doesn't seem to be short stroking, it doesn't eject the spent round and then it attempts to load the spent round as well as a new one, it really smashes up the spent casing really well.....

Manually cycled a few rounds through it today after re-assembly, a couple times it failed to drag a new round all the way out of the mag, it would eject the rounds and cycle new ones if I was fast and hard with the action, didn't seem to have a problem pulling new rounds at the range so I think it was just me being slow with the bolt..

If it's locks th ebolt back then you have a extractor problem.  A O ring or D ring upgrade is in order, spring and insert too.

Thanks for the help, I have a spring and o ring comming in the mail. I'll post with results!

It sounds like the extractor is holding onto the rim just fine, maybe even too much. I would try to go the other way and remove the o-ring, or use a lighter spring.

Maybe the ejector might be the problem. Weak or damaged ejector spring maybe? Or maybe a bur on the ejector causing it to bind up? I don't really have a lot of experience diagnosing problems with the AR platform, but I don't see how increasing the extractor pressure is going to help anything. Seems to me like it would make it worse.

With the higher pressure and very short
dwell time of the short pistol barrels, the case never really has enough
time to shrink back down before the extractor is trying to pull it from
the chamber. Adding a O-ring around the spring increases the spring
pressure allow the ejector to get a better bite on the rim. O-ring
generally don't come installed so chances are there he cannot "go the
other way and remove the o-ring".
The o-ring is done by NSWC-Crane on the short
barreled CQBR and Mk-18 platforms to counter exactly these problems.





 

Right, but the extractor in the OP's gun is not having a problem getting the case out of the chamber. The gun is having a problem getting the spent case out of the gun and it's trying to jam it back into the chamber along with the next round. Is this also a common result of a weak extractor? I'm not arguing, just asking a serious question. It seems more like a problem with the ejector to me. I feel like people are stuck on the "failure to extract" problem when the gun is actually "failing to eject." I apologize if I'm off base here. As I said, I'm not an expert. I'm just trying to make sure the advice given is for the correct problem here at hand.

I had the behavior that the OP describes, and it was fixed by installing an new extractor spring and o-ring (I didn't have the o-ring originally).
It is hard to say exactly what is occurring, and failing to extract does also result in a failure to eject.
The spent case is coming out of the chamber at least partially, and may in fact be riding in the extractor until a new round dislodges it.
Why would the extractor have purchase on the spent case long enough to pull it out of the chamber, but then release it before ejection?
Could it have a weak grasp of the spent case, and the attempt at ejection causes it to be dislodged but not ejected?  
If the extractor never had a grasp on the spent case, would the spent case get pushed out of the chamber by gas pressure?

Whatever was happening, stronger pressure on the extractor (due to the added o-ring) seemed to fix it.  I used to get FTE every 3-4 rounds; since adding o-ring 500+ rounds without issue.

I had the same issue with my pistol and an o-ring and 9mm buffer took care of it.not a single jam since about 600 rds.

I appreciate all of this info, and espically the debate hahaha! But again, all very useful. I found a different bolt and extractor set up I am going to try and see what happens there, also o ring is on order as well so either way I should be able to remedy my problem, I hope so anyway. I will post results and if I can figure out how to pics as well! Thanks guys!