When the barrell is screwed into the receiver, it should come out with a line extended straight up from the front sight that is 90 degrees with the plane of the upper receiver.
It's a little hard for me to explain, but if you were to put a level across the flats of the upper receiver with the dust cover removed, and run a long straight rod thru the hole in the gas block where the hand guard screw goes, the level and the rod shold be parrell.
Not sure if I explained it well enough. I know in my mind what to look for, but can't explain any better with out pictures.
Any, that is barrel timing. If the barrell is not screwed in enough (undertimed) the front sight post will be canted to the left. If the barrell is screwed in to far (overtimeed) the front sightwill be canted to the right.
Try to set the rifle in a vise or some sort of stand. Position yourself behind the rifle and sight down the barrel. If the barrell is over or under timed, you might be able to see the cant of the front sight post.
If it's too far one way or the other, the gas piston will bind. That's not the only problem that will occur, but that's one of the symptons.
Check out www.falfiles.com/forums and go to the gunsmithing & build it yourself discussion. From there you can do a serch of the archives and there is a wealth of information there. You may have to wade thru some BS, but the files have been a great help to me in my 6 buillds.
On one of my builds, I had a gas tube that was out of round. I ended up putting the gas pistion in backwards to find out where is was binding.
I'm including a picture of the timing tool I use and how it's set up.
Keep plugging. Ken