I would venture to guess that a spindle on a cannon carriage would not be hollow. I think the weight would have been to great. That said, I can't figure out why any spindle would be hollow with the exception of maybe being easier to produce by a black smith. Nice find.
“I won't be wronged. I won't be insulted. I won't be laid a-hand on. I don't do these things to other people, and I require the same from them.”
― John Wayne "The Shootist"