It has to be the gas tube, I have eliminated every other possibility systematically over the last several outings. I was just out last weekend and would fire and the bolt would go back about 3/4 and stop, a pull on the charging handle, fresh round, same thing. Did this about 10 times just to be sure and then put it up.

So can the gas port mod be done with simple tools or do I need to take it to a gunsmith? Oh by the way it is an 11.5 with a 5.5FH soldered in place so I can't get a different muzzle for increased back pressure. I am almost at the point of just replacing the barrel, selling it and being done with it, but thought I would take one more step and see if I can get some realiability out of it.

I'd replace the bbl, unless you just like the look of the long flash suppressor.  That bbl configuration doesn't do anything for me.  You still have a 16" long bbl, and the velocity performance of an SBR.  I have a 11.5" SBR, and it has it's purpose.  I'd get a 14.5" bbl with a permanently attached flash suppressor, or a 16" bbl.  You'll like the longer configuration better, and it will allow the actual bullets to fragment out to longer distances.  With the 11.5" bbl your bullets won't fragment much past 50 or 75 yds.  They'll still put hole in things out to 600 yds, and I still wouldn't want to get hit by one.

Are you sure the gas-key isn't loose? I know you said you eliminated other possibilities, but I just wanted to throw it out there in case you forget about that one.

Is your FSB off center... partially blocking the gas port?

No to a loose gas key and no to the FSB being off center. I am inclined to think the gas hole needs to be widened, but just how much I am not clear on. Replacing the barrel is an option and it really comes down to money as to do it is pretty easy. But essentially I am doing it anyway because having the FH to get it too 16"......

I built a "cheap" version along the lines of the XM's with the 11.5" and the 5.5" flash suppressor but with a flat top. I had many similar problems. I scoured the AR15.com and the net and tried all the "easier" fixes. Finally (I am pig headed and HATE to take/send stuff back) I removed the barrel, removed the front sight and shined a bore lite down the barrel. The gas tube wasn't drilled through. I drilled it out right at .085 and it works beutifully. There is a chart at: http://groups.msn.com/TheMarylandAR15ShootersSite/maintenacefaq.msnw that has the minimum and maximum. This all took me about a month and 5-6 trips to the range. I did also have a problem with a slightly loose gas key. Good luck.

What ammo are you shooting?  I have a 12.5" 6.8SPC with a KX3 that did the same thing with Hornady ammo.  When I switched to SSA, it was nothing but reliable.  Your ammo may not be "hot" enough to provide the correct pressure.

If you're going to have a barrel with an overall length of 16", you might as well get yourself 3 more inches of barrel and go with a 14.5"  

What brand is it? What buffer are you using and what ones have you tried? How did you come to the conclusion it had to be the gas tube?

Never said "gas tube" but hinted towards to the gas port hole.

Ok, but still, what brand upper are you using? How did you come up with the elimination that it was the gas port? What buffers have you used? How is the staking on the gas key? Have you removed the gas key? How are the gas rings? Did you do a function test to determine if they were good or shot? Is the fired case extracted and ejected or do you have short stroking jamming the fired case underneath a new one? I understand that you have taken it out over several outings and eliminated other possibilities. WHat were the ones you eliminated and how?

There is much diagnostic information that is not given. Did you measuer your gas port? The bolt only going back 3/4 of an inch just tells me something else isn't working right, not necessarily the gas port. I'd hate for you to drill and have that not be the correct solution.

Ok, but still, what brand upper are you using? Bushmaster How did you come up with the elimination that it was the gas port Everyone tells me this is 90% of the time the reason? What buffers have you used?I have used a standard buffer and now have an H3 in it. The regular buffer I am assuming a H1 did not work, had the same issue then purchased an H3 and still have the issue How is the staking on the gas key?Not sure of the term it escapes me, but it is notched and I have removed it before and put locktite on the threads to ensure it will not move Have you removed the gas key? How are the gas rings?I purchased a new bolt (same carrier) from Bravo Company, so it has new gas rings, got the improved extractor ring, everything Did you do a function test to determine if they were good or shot?It is the same bolt configuration that I use in other AR's I own, but yes I took the bolt out to verify it was not something malfunctioning with it Is the fired case extracted and ejected or do you have short stroking jamming the fired case underneath a new one?The bolt itself will come back about 3/4 of the way, not full extraction or will eject but not be back far enough to take a fresh cartridge I understand that you have taken it out over several outings and eliminated other possibilities. WHat were the ones you eliminated and how? I used the old buffer, got a new one. Utilized an old bolt, bought a new one, tested it in other rifles, still no go. Tried different mags, different ammo (some wolf, some UMC, some Lake City, some Barnaul

There is much diagnostic information that is not given. Did you measuer your gas port?I have no idea how to do this let alone the correct tools in which to do it that I am aware of.... The bolt only going back 3/4 of an inch just tells me something else isn't working right, not necessarily the gas port. I'd hate for you to drill and have that not be the correct solution.Me too

Quoted: Not sure of the term it escapes me, but it is notched and I have removed it before and put locktite on the threads to ensure it will not move

You should read up on this. Loctite will not do. The gas key and carrier will get too hot and will make the loctite usless. The gas key screws needs to be properly torqued and they need to be stake to ensure they do not move at all. Any gas escaping between the carrier and gas key can cause problems.

Why did you remove the gas key in the first place?

Did you build this upper yourself?  What type of gas block does it have?  The gas block and the hole in the barrel could be mis-aligned.  Most barrels have a step in the barrel just behind the gas block.  But if you have a low-profile or set screw gas block or one that doesn't use taper pins you can get a mis-alignment of the gas hole and gas block.  If the gas block was installed and seated all the way against the step in the barrel behind the gas block area this could cause mis-aligment.  There usually needs to be a space between the step in the barrel and the gas block.  This is for the piece of metal that allows standard handguards to be held in place.  

Could you post some pictures of your upper?

I removed the gas key because where does it ever say that you can not or shouldn't? Maybe for basic dissambly there is no need, however I am kind of clean freak and I took it off once after oh I don't know several outings to ensure this was not the issue and to see if it was tight, etc. Right now it has not gotten hot enough to do anything to the loctite, because I can not get the thing to fire 2 rounds in a row.

That is something good to check, I will check the gas block and front sight base. I know it was removed by a smith to attach the 5.5 FH, so I will need to check this out to ensure it was put back on....I don't think he test fired the thing when he did this.....I have had realiability issues before, but not this bad.

Quoted: I removed the gas key because where does it ever say that you can not or shouldn't?

Page 140
www.ar15.com/content/manuals/TM9-1005-319-23.pdf

The carrier key screws are normally staked to prevent them from vibrating loose. Some can still be removed after they have been staked(improperly) but most of the time you have to grind the carrier key down in order to get the key off. Then you would replace the key and screws and reinstalled using the methods in the PDF above. If they weren't staked in the first place, then it could have been your problem a along.

A staked carrier key should look something like this:

You said you tried a different bolt, did you try a different carrier as well?

Assuming the gas key isn't the problem,(though you should get it done or do it right before it becomes a problem) I have a couple questions about the gas block:

Is it a clamp on, set screw or is it a standard front sight base with taper pins?

Was the hand guard cap removed?

Like the other poster stated if it's some kind of clamp on gas block and the hand guard cap was removed for a free float rail, then there is a possibility the gas block is too far back(toward the upper). There should be a 1/16th of an inch or so gap between the gas block and the step up where the cap would usually sit. If the block is too far back it could cause issues.

One thing to note in the pic of a properly staked gas key is that the stakes are actually enough to indent in the screws. Many manufacturers barely make a dent.

The gas key being loctited could be a reason why. As to the size of the port, it would involve taking out the pins for the front sight block, the pin forthe gas tube, and then sliding the assembly forward to expose the gas hole.