did you acquire the rifle new, or is it a pre-owned rifle that is just new to you? if you got the rifle new, any of the documented barrel nut removal methods should work. if it was a pre-owned rifle, the previous owner "may" have used some sort of thread locking agent on the barrel nut, in which case you have your work cut out for you if you do not want to damage the upper receiver...probably the best method then would be to use an impact wrench with a special modified barrel nut tool.