Chris, yes, a solid mandrel does support the barrel. Had a senior moment there.
Though I am too fat, old an out of shape to weld a barrel, the smith I have most often seen demonstrate forging barrels doesn't like using a mandrel, because they tend to stick in the bore, so in this case, the V allows the skelp to be welded without collapsing the bore.
Bob prefers to forge the skelp around a 3/8 or 1/2" dia steel tube, instead of the mandrel. The tube is drilled out during drilling and reaming, so nothing remains of the tube once the bore is prepared.
Bob is an accomplished smith who briefly tried to forge barrels commercially, back in the 1970s, I think. Since he was interested in production instead of historical accuracy he modified the process by eliminating the mandrel and using the V to support the barrel during welding, among other things. In this case, without the solid mandrel, using the V does keep the bore from collapsing.
Bob's attempt at forging barrels commercially failed, in part, because a vast majority of ML shooters tend to be too cheap to buy a forge barrel. Not to mention that no commercial barrel maker would drill his blanks. It seems that the barrel makers would only drill solid stock, as opposed to a relatively large dia barrel with a small dia hole, which added a complication that he could not overcome without a significant outlay of cash.
I have also watched him forge a pattern welded pistol barrel that came out real nice. Real, real nice.
God bless