No, the receiver is, but it can be casted like the originals were. Trouble is the metallurgy and it must be strong but ductile enough so there's no stress cracks like some of the oriignals were. Was speaking with a summer student who is an aircraft engineer who knows metallurgy and he's thinking of writing a metallurgy handbook for gunsmiths. I told him to go for it and self-publish so he doesn't have to have boxes of books laying around and try to sell it himself. I did show him the Girandoni construction book and he's intrigued.
If you go to Beeman's site, you'll see a modern (circa 1940s) tubular airgun that was inspired by the Girandoni. The big difference in the magazine tube is that it is very much like that of a lever action. It has an internal spring and follower so there's no need to tip the barrel up to reload it. Just depress the feed button/block and the spring loaded follower automatically pushes a ball into the loading block.
You could make it like the 1860 Henry where there's a tab on the follower that allows you to push the spring back (and with a notch in the magazine tube to hold the follower back) and reload the magazine from a reloading tube, close the loading latch and release the tab (probably gently) to allow the follower spring to decompress and push the follower forward. Of course like the Henry there is an issue of dirt getting into the magazine tube and then into the action. There's a reason why there's a loading gate on lever actions (like on the 1866 Henry).
I'm wondering if there's a way to adapt the Spencer rifle's system to the Girandoni? The Spencer was butt fed and you removed the self-contained spring & follower to load it. This would require more metal (another tube with follower & spring) that is inserted into the magazine tube after you deposit the balls from the charging tube. This means more parts & increased weight too.
