Don't laugh at me. I'm thinking of tackling my first upper assembly. So, obviously the gas tube has to be removed so I can set the barrel extension in the upper receiver and then proceed to tighten the barrel nut.

My question is can I remove the gas tube from the front sight assembly and then reinstall it after the barrel nut is tightened and aligned or does the front sight have to come off as well?

It seems like because of the bend in the gas tube, I won't be able to feed it after the fact through the receiver and nut and re-pin it in the front sight assembly. Having never done this before, I could be wrong. I hate the idea of knocking out the front sight base pins, but I will if I have to.

if you remove the roll pin from the FSB, you should be able to push the gas tube far enough into the receiver in order to remove it from the FSB; once that is done, just slide it forward out of the receiver. It should go back in the same way, insert into the receiver far enough to clear the FSB, then insert into the FSB and install roll pin

 

here you go. Brownells video
they have videos to assemble an entire AR.

So, just to be clear what you're saying is remove/unpin the gas tube and install the barrel with the front sight base intact. Then, slide the gas tube through the receiver toward the front sight base and repin into place?

Okay, that's interesting. He left the front sight base installed and inserted the gas tube through the front of the receiver. I figured that wouldn't work because the front sight base would be in the way. It didn't really show what he did but I'm assuming he angled it in there enough to clear the front sight base and then slid it forward into position to be pinned.

And you would be correct !