[Walt Fricke]

While you resolved this small part of your problem, here for your future use and the good of the order is my tip for rounded internal hex bolts or barrel nuts (if you can't access their outsides):

Use a punch or small chisel to distort the sides of the hex. You can pound enough of the metal you stripped away, plus what was always there, back in to the point where your hex tool will bite. You may have to pound the tool end in, but that is all to the good, as you are reforming the hex.

This can be especially useful on these head stud barrel nuts, because often when torqued the end of the stud gets awfully close to the top of the nut, leaving little to grip, making rounding easier, etc.

Walt