I’ve read the last few replies on this thread and thought that it would be a good time for me to add some barrel-smith expertise to the comments.
On the topic of mandrels, just to be clear, the mandrel used to wind the riband into a coil and the mandrel used during the barrel forge welding are two entirely different tools.
As to taper on the mandrels, the mandrel used for forging is tapered. The reason for the taper is to make it easier to remove it from the barrel tube after forging. During forging, the barrel tube is forged down tightly on the mandrel. It often takes a firm rap with a hammer, or other method, to break the mandrel free of the barrel. Once it starts to move slightly, the taper makes it possible to pull it from the tube freely. If it were not tapered, it would have to be driven its entire length past the constriction in the barrel bore.
The mandrels that I use for winding the riband into a coil are also tapered, though not so much as the mandrels that I use for forging. As the riband is wound, it is wrapped tightly on the mandrel. This can create a great deal of resistance against removing the mandrel. So here too, there is a benefit to having a taper on the mandrel. I cannot say whether or not the mandrels that Pete saw were tapered, as I did not see them. It may not be essential that winding mandrels be tapered, though it can prove beneficial. As the winding mandrels do not need as strong a taper as forging mandrels, perhaps the taper may not have been clearly evident along the length of a four foot long mandrel.
Concerning the chemise; I can state with certainty that the chemise is not used to prevent the mandrel from welding into the barrel. The welding process creates no risk of the mandrel being welded in this manner. Note that the British barrel makers did not use a chemise; at least not during the later years of production. This can be seen in the demonstration tube photos from the Birmingham Proof House, posted earlier in this thread. Clearly, the British makers had no fear of the mandrel being welded.
Jean Puraye stated that the chemise was to facilitate the entry of the mandrel into the barrel. Again, no need for this, as there is no difficulty involved in inserting the mandrel. The British apparently agreed that a chemise was not needed here.
I cannot honestly say that I am certain of the purpose of the chemise, though I have a theory, based on what I believe it could do for me in a barrel welding operation. The forging mandrel, when placed into the barrel is cold. The mandrel is never left in the barrel when the tube is in the fire. The mandrel must be cold, to provide a solid anvil surface to form the inside of the barrel bore. As the mandrel is cold when inserted into the bore, it immediately begins to draw heat from the barrel tube. This can have a very negative effect on the welding of the barrel tube. Forge welding temperature spans about a 100 degree Fahrenheit temperature range. Once the material is at welding heat and is removed from the fire, it begins to lose heat. The smith only has 3 to 5 seconds to close the welds before the material slips below the temperature required to make the weld. Shoving a cold piece of steel (the mandrel) against the area to be welded will shorten the available time to close the weld. I believe that the chemise serves as a thermal buffer, to prevent the cold mandrel from robbing the heat needed to close the welds between the turns in the riband.
Pete is correct about the possibility of chards of chemise coming loose inside the barrels. I believe the barrel makers wished to see the chemise become solidly forge welded to the inside of the tube. However, referring back to the effect of the mandrel robbing the welding heat, it sometimes fails to weld completely. The bore finishing process is intended to completely ream the chemise material from the inside of the barrel. However, the barrel makers of old also had to deal with the meandering forged bore hole. So occasionally, the entire chemise would not be removed. If this occurred where the chemise was not solidly welded, the chard could be stripped from the bore during firing. W.W Greener called this, “throwing slivers”.
