That is very unusual. I build AR uppers and the specified torque for a barrel nut is 35-45 ft lbs. It should not be that tight. They may have used a thread locker on it so you may need to juice it with a penetrating oil and some heat to break the seal if that is the case. Be careful---I've seen where the nuts were so tight that it sheared the barrel locating pin off and allowed the shank to spin---that would be a bad thing to occur. Also, they should be using an anti seize compound on the shank and barrel nut threads as a steel nut could become bonded due to dissimilar metals (Aluminum receiver and steel nut).