I just finished a 16" Hbar, and I need your help fellas.  I put this BM 16" bbl on a Colt A1 upper, and an Eagle arms lower.  The other parts had been used before since I have put a thousand or so rounds through this gun when it had a 20"  A2 Colt barrell on it.  The 16" bbl assy is the only new part.  

Now it will shoot the first few rounds like a single shot, and eject the empties, but slams shut on an empty chamber and has to be manually charged to fire.  After about 30 rounds like this, it warms up and runs just fine.  It will do this with any kind of mag.

I did a gas ring check from the Army manual, checked the gas tube for center, checked the carrier key for tightness, and even put in a new tube, but it didn't help....

any suggestions...???

Thank you in  advance

greco

Tag.  I am experiencing a similar problem.

Well, this is not a cure, but I feel better....This problem has been occuring with a full size rifle stock assembly.  I removed the lower and put on a lower with an old 3 postion telescoping carbine stock.  I took it to the range this afternoon, and it worked flawlessly.  

Obviously, I didn't fix anything.  My 3 other M4 style uppers all work just fine with any kind of stock.  This one seems to have a problem with the fixed stock.  The rings seem to be as tight as in any other AR I have.  

Maybe I will change rings just for the heck of it.  Maybe I'll change the entire bolt/carrier and see if that is the difference.  For now, I'll just leave the tele stock on it.  Is it possible that the spring and weight in the fixed stock is too much for the gas system to overcome..??  If that is so, why do all my other AR's work just fine with either stock.  None of them is a excessively worn.

Well, I removed the old gas rings from the bolt, and man, were they worn thin...!!  This gun has less than a thousand rounds thru it.  It also passed the military test of holding the carrier and seeing if the bolt will fall out on its own weight.  Damn, when I put the new rings on, the tension increased a whole bunch.  Like three times harder to push and pull the bolt in the carrier.  I'm going to the range later this week....I bet you I found the problem.  I think I will buy Lots of rings and keep them handy.  I can't believe the rings wore out on this Colt assembly so fast.  I'll let you know....

If your bolt closes on an empty chamber then you either have a problem with your magazine or your bolt stop.  When the magazine is empty the back end of the magazine carrier should come up under the bolt stop and lift it up to stop your bolt.  Perhaps the bolt stop is binding in the lower until it warms up?  Is the front of the bolt stop worn?  Does the magazine carrier contact and raise the bolt stop when the magazine is empty?

JCUMM2, what is happening is that the bolt slams shut on an empty chamber when the mag is full. The bolt doesn't seems to be going back far enough to grab a new round off the top of the mag.  You then have to grab the charge handle, pull it back, and let it feed a live round.  Then you pull the trigger, the spend case weakly ejects, and the bolt slams shut again on an empty chamber, yet the mag still has 26 rounds in it.  After a few shots, it warms up, and works o.k.  Every other gun I have works o.k. with either the full size stock, or the telescoping carbine stock.

By the way Jasoncap, what ammo are you using??? Is it milspec..???  White box..?

greco - I have tried Winchester White Box 62 grain, Federal 62 grain, Hornady TAP 75 grain, and Federal Gold Medal Match 75 grain all with the same results.  I have used (5) brand new LeBelle 30 round mags, and (1) new Bushmaster 10 round mag.  Again, all with the same results.

Wow....took ailing AR out this week end.  I had installed new rings to see if this would help, even if the old rings were within spec.  I ran 3 rounds...everything...o.k..let it cool 1/2 hour, ran 5 more rounds...everything o.k.......let it cool 1/2 hour, ran 120 rounds.all flawless....ejection was great, feed great. Worked with short or long stock.  It was definitely the rings....I'll have to remember that the next time I have ejection problems....

Are you listeneing Jasoncap..????

Thanks for the tip.  I may go ahead and order some new rings and see if that is the problem.

Quoted: Well, I removed the old gas rings from the bolt, and man, were they worn thin...!!  This gun has less than a thousand rounds thru it.  It also passed the military test of holding the carrier and seeing if the bolt will fall out on its own weight.  Damn, when I put the new rings on, the tension increased a whole bunch.  Like three times harder to push and pull the bolt in the carrier.  I'm going to the range later this week....I bet you I found the problem.  I think I will buy Lots of rings and keep them handy.  I can't believe the rings wore out on this Colt assembly so fast.  I'll let you know....

I hope you've found the culprit, but sometimes gas rings will mask a bigger problem. A thousand rounds shouldn't have worn the rings out--heck, the rifle is just barely broken in.

It sounds to me like you're not getting enough gas. A rifle buffer is heavier than most carbine buffers, which makes it harder for the light gas pressure to cycle the bolt carrier. Since the only thing you've changed is the barrel itself, that pretty much narrows it down to the gas port or front sight base alignment.

If the gun failed to function with the new rings...my money was on the gas port being too small, or blocked by something (a burr, or undersized).  I even put a clean, new tube on just to be sure.  Luckily it worked.  I will keep an eye on this to see what happens when the gun has a higher round count on it.  If it starts to get iffy on me, I'm gonna take the barrell off and send it to BM to have them check out the gas vent hole and sight allignment.  Better them than me.  

I had the exact same thing happen with my carbine.  I put a full size stock on during the awb and it would not run for anything.  I put the carbine stock back on after the awb and it works flawlessly.

Not sure if you could just run with the carbine buffer in the full stock buffer tube or not.  There was a post earlier someplace that listed different weights of the various buffers.  But I would have to agree that the gas port may not be large enough for the heavier buffer.

So, I suppose you could do one of 2 things. 1) enlarge the gas port, or 2 run with a lighter buffer.  Someone correct me if I am wrong here.