If the barrel is a common caliber, come up with a piece of brass that has been fired a few times with a good stout load, Size it with your dies and shell holder, with the die touching the shell holder during the sizing process, with no toggle,making sure that the shoulder was bumped in the process, and then use the resultant case, with a couple of layers of scotch tape on the back, to set your barrel nut barrel. Of course a go gauge would be a lot better, but that should get you there, and you should be able to size your cases with your die and shell holder.