Mike,

That is the chemise, not the mandrel. The chemise was a sheet of iron that was wrapped over the mandrel to keep the barrel from being welded to the mandrel.

There are early accounts of poorly made barrels where shards of chemise would become jammed in the barrel, causing the barrel to explode. To my knowledge none of those barrels has survived... But they certainly contributed to the period reports of cheap Belgian guns.

That picture resembles the Baker example barrel.

Pete

http://www.damascus-barrels.com/Movie.html