unless there is a bump at the end of the threads the nut should continue till the threads run out. put your nut wrench on the nut and just using your hand see if it will screw on some more. it could be a rough spot on the threads (not uncommon on a new barrel) and the nut just needs to get past it. if it is a rough spot this will take care of it. remember to lube the threads before you put it all together with a touch of neversize or a spot of gun grease.

Gary