I have a 11.5 upper with a perm. attached ak muzzlebreak. The thing will just not function right , it will only fire 2 or 3 rounds before closing on an empty chamber. I have replaced the buffer with a bushy heavy buffer , replaced the ejector and spring and installed a D-Fender , I have also put in an M16 pigtail . This has a retractible stock , it is a pre-ban . I have tried several different mags , ammo , just about at my wits end... Any help would be welcome

Check the carrier key, do not look at it to see if it is tight, put an allen wrench on it and turn it till it is tight, i had an ar do this to me once, the carrier key looked good, but when i finally tried to tighten it with a wrench, hmmm, problem went away, a carrier key loose, even ever so slightly will cause a gas leak, and cause the gun to short stroke, let me know what you find, also check to see that when you close the bolt, the carrier key lines up perfectly with the gas tube, there should not be any drag on the key/tube

I checked the carrier key and it is tight , and the key lines up with the gas tube . Everything seems to move without binding. I think I may try a different carrier assembly , if that does'nt work I guess I'll just have another parts gun. Thanks..

i bet you already have checked the gas rings, and the bore where they ride in the carrier

The over ride may that the mag catch needs one more turn on it?

Okay,a little more info on whats going on with this thing. The rifle has about 40 rds. through it , it started out only firing singles,then it would try to feed 2 rds. at a time, bad mags, fixed.Then it was short cycling, replaced gas rings, installed M16 pigtail,heavy Bushy buffer,Bushy ejec. and kit, and D-fender. Checked the carrier key, OK. I did everything but open up the gasport to .081, don't want to go there. Talked to the Bushmaster armorer????Everything checks out as it should , I'm thinking maybe an Exorcism , Help!

I had a 16 inch that won;t cycle right either. Tried everything I could think of.

After an AR expert tuned it properly, it worked fine.

Problem? Gas Port not jetted properly...too big, allowing too much gas...needed a small port plug installed.

I have heard the shorter barrels are notorious for not cycling right.

You need an AR Smith who can work on short-barreled AR's and guarentee their work.

Lotsa Luck finding one!  

AMAC, Shorties get mentioned a lot for either short cycles or leaving the empties in the chamber. The heavy buffer and the Pigtail and the D-Fender are all fixes for leaving empties in the chamber, but your rifle is not doing that. By the way, these changes will help fix having too big a gas port too.

It sounds like it is short stroking (Short recoil in the TM). First off, has the chamber been polished yet? Even a light polish with a chamber mop and Flitz will help a lot of rifles achieve reliable extraction.

Next, are you using Greek surplus? It has been noted for doing this with otherwise reliable rifles. Next, have you cleaned it since new? Some rifles are pretty tight when new and generate a lot of crud. Remove the carrier and buffer/spring, clean the carrier, buffer and inside of the upper. Next look over ther buffer and spring for any binding of coils over the buffer. Reassemble and check for any binding - that can suck up the energy that issupposed to go into a full stroke.

After that get back to us. You may eventually have to open the port, but let's make sure that everything else is done first because putting metal back on after opening the port is tough to do...

Billski,  AMAC is all yours.

After reading his post, he has compounded the problem, and not fixed a single item that should have been checked.

My only suggestion would be for him to pull all the add-ones,get the rifle back to stock and break the rifle in with a nice coat of CLP to aid the rough surfaces in polishing themselves.  This would also include a hammer that may have a tail too long to allow the carrier to ride over the hammer without binding on the back of the disconnetor.

Note: The one part of really loved is since his buffer is not coming all the way back to pick up a new round,  he added a even heavier buffer to make the buffer to come back even less, then added a Pigtail gas tube.  I guessing that his carrier barely moves now.

AMC, Someone made a lot of money selling you items not needed.  With any luck, you can get a refund on the parts or find someone who needs the parts and will buy them from you.

As Billski stated, get the rifle back to stock, then try running the rifle with a fresh coat of CLP to assist the rough carrier to glide past the rough upper surface.  Also, the complete rifles bearing surface(upper) are new and rough.  The CLP will keep the new parts from binding on each other until they polish themselves out.

Just food for thought.

Everything I tried was one step at a time and it seemed to improve the cycling problem. The parts I added were parts I had on hand since I have worked on several of these little gems. I have tried USA , REM,LAKE CITY, etc.. ammo , and it does not seem to matter. Think I will just keep plugging away and see what will work , this baby has got me stumped.. Thanks...

AMAC,  The only thing that is going to fix your rifle is time.  The rifle is tight and is binding.  The reason that each one of your changes "SEEM" to make a difference is that every time you pulled the rifle apart you cleaned/polished  the rifle a bit more.

Put the rifle back to stock, then go to the check list. It's the first post in this forum, and was posted by Tweak the Mod.

Check the gas system from the barrel port, all the way back to the gas rings to confirm that you have flow.  

Check the FCG for binding.
Pull the buffer and check for binding.
And, keep the upper wet with CLP until carrier/upper have mated.

Hope this helps.

Is this a Bushmaster by any chance? I bought a bushmaster 20" in Jan. and was showing a serious short stroke problem. Also bad jamming problem. Didn't modify it but checked everything and come to the conclusion that since the windage had to be almost all the way to the left and was close but not right on, that the front sight post wasen't indexed properly and maybe obscuring part of the gas hole or the hole was out of spec, so took it back where I bought it to be replaced (it was covered by this Davidson-D Guarentee thing). Been 2 months and still waiting. My 16" RRA that I built from scratch shoots flawless even with the Greek m193 mentioned above.  

Hey AMAC, I thought that I had replied with my next level of comment, but it must have gone into the ozone.

First, don't let the chatter on here get to you.

Now, have you polished the chamber yet? I consider that essential to reliability in autoloaders. Even some Flitz on a mop and turned with a drill will help.

Next, slip the front sight housing and check for evidence of the gas port in the barrel being lined up with the passage in the housing. There should be witness marks on the barrel from powder ash showing where the hole in the housing was. If the holes were choking each other, it will cause this.

This is a good time to put a standard gas tube back in for the purpose of trying it out.

In honestly, I would go with a heavy buffer in carbines as my starting point. That is what the GI shorties and M4's use. I have ballasted both of my competition guns (epoxied a sleeve of stainless steel in the rear end of the carrier) to slow down the action a touch. It is easier on my pampered match cases and works fine.

Anyway, when you have checked the stuff we have all suggested, then we can move on to more serious steps.

Hey AMAC, I thought that I had replied with my next level of comment, but it must have gone into the ozone.

First, don't let the chatter on here get to you.

Now, have you polished the chamber yet? I consider that essential to reliability in autoloaders. Even some Flitz on a mop and turned with a drill will help.

Next, slip the front sight housing and check for evidence of the gas port in the barrel being lined up with the passage in the housing. There should be witness marks on the barrel from powder ash showing where the hole in the housing was. If the holes were choking each other, it will cause this.

This is a good time to put a standard gas tube back in for the purpose of trying it out.

In honestly, I would go with a heavy buffer in carbines as my starting point. That is what the GI shorties and M4's use. I have ballasted both of my competition guns (epoxied a sleeve of stainless steel in the rear end of the carrier) to slow down the action a touch. It is easier on my pampered match cases and works fine.

Anyway, when you have checked the stuff we have all suggested, then we can move on to more serious steps.

Make sure you have the correct recoil spring.  The fixed stock spring is 44 turns 12.5" long and the collapsible stock spring is 36 turns 10.5" long.

Good comment too by oldguy.

One other of my favorites: Look over your brass for signs of excess pressure - cratered or pierced primers, brass extruded into the ejector opening, blown primers, etc. High pressure rounds extract hard, and could be from small chambers, short throats, etc.

AMAC:

Browsing and read your post. Don't know if you have fixed your problem yet but if not check the gas tube on the bolt carrier. identical problem with a student's gun not to long ago. had a primer stuck in the gas tube on the carrier. cordless drill resolved the problem in less than a minute.