You don't remove it, you just use a worn barrel brush and pipe cleaners to ream it out.speaking of cleaning the gas tube, how is that done & are special tools required to remove/re-install it?
To the OP, make absolutely sure that you have all of the carbon out of the rifle. Other than that, you've got to get to know the rifle and figure out if it likes heavy oil, light oil or no oil at all. There is a reason that you have an assigned rifle in the military, this is the main reason.
What are you using for oil? I recommend Break Free CLP, as I know that is what works.