My barrel nut has become slightly loose, and needs tightening. Unfortunately, I cannot seem to be able to remove my gas tube. After removing the roll pin that retains the gas tube in the FSB, the tube will not loosen or move from the FSB. I tried using pliers and tapping them with a hammer, as described in the tacked thread in the Building section, but the tube won't budge, and I now have a scratched tube for all my efforts.  I am worried about using too much force, and deforming the tube, but  I don't know where to go from here. Any suggestions on how to remedy this, short of removing the FSB? The barrel is a 14.5, 1/7 CMMG w/perm. Phantom, if that matters.
TIA

I had the same thing happen today.

I used some penetrating oil around the tube to FSB connection and stood it up so it would get in there good.

I then took a small pair of channel lock pliers and gripped the tube, started twisting (gently) the tube while pushing it back towards the upper.

Worked like a charm for me. I polished the marks off the tube later with a fine stone.

Danny

The roll pin hole is soaking in WD40 as we speak, I'll give it a few min. and try your suggestion. Thanks!
Any other ideas?

Vise barrel up, wrap a piece of leather around the tube, then grab the protected tube with a pair of pliers.  Now instead of tugging, just hit the side of the pliers with a hammer to jar/start the tube out of the FSB.

P.S. Easy on the pliers pressure He-Man, too tight and you’re going to crush the tube.

Stand it on the barrel, take out your charging handle and bolt carrier, then try to tap it out from the upper end of thing with a wood dowel.  If it comes out make sure there is no wood in the tube.  Oh, ya and lube the heck out of it and check the barrel nut alignment it might be hanging up on the nut.

Even after 700+ rounds, my gas tube came out using my fingers...

Try rotating it first and then pulling it out.

Thanks for all the replies.
After my original post, I propped the upper, barrel down against my workbench. I liberally sprayed WD-40 into the rear of the FSB, where the gas tube exits. After allowing the oil 15 min. to do its job, I placed my upper in an upper receiver block in my vise. Using a pair of small pliers, and a section of innertube wrapped around the gas tube, I tried twisting and pulling the tube, then using the technique with the hammer and pliers all to no avail. I am being careful in regards to the gas tube, checking it each time to make sure it is not deformed. As far as I can tell, the tube hasn't even budged. I checked the tube at the barrel nut, and it doesn't appear to be hanging up on it .
The upper is still soaking in WD-40 and I'm going to have a crack at removing the tube again later on today.
Tweak- I have a vise, do you have any suggestions on what I should do?
Any other suggestions on what I should try now?
And thanks again for all the replies, I really appreciate the advice. (even if it hasn't worked out )

Heat up the front site post with a torch.  It should come out when it's good and hot.

Quoted: Thanks for all the replies. Tweak- I have a vise, do you have any suggestions on what I should do? Any other suggestions on what I should try now? And thanks again for all the replies, I really appreciate the advice. (even if it hasn't worked out )

From another thread

Quoted: clamp the tube in the vise hard enough to hold it but not crush it. smooth jaws work best or use a jaw cover. pull the barrel off the tube.

Well I finally got the gas tube out, using the method described by Tweak in John-NC's post above. It took about 15 min., and a lot of twisting. It came out fine, I tightened the nut, and re installed the tube without a hitch. Thanks for the assistance.  

get one of these. Gas tube wrench, very helpful