The rifled tubing idea sounds like a winner to me- a lot easier to cut a chamber than a rifled barrel.
However, it IS possible, even plausible to cut your own rifling. Cutting into aluminum would make it even easier.
There's a series of books called "Foxfire" that discusses life in the "old days". One of the books (foxfire two iirc) goes through the history of old time flintlock rifle makers in Tenesse (et al). They have several guys who've actually rifled their own barrels (and customers barrels) and they show you how to do it. Not tough, nor (with an aluminum tube) would I think overly time consuming. Best of all, you can make the rifling equipment yourself for next to nothing.
Kevin, I can lend you my book if you want, although they're pretty cheap and available online- and I bet once you see this, you'll want your own.
Cheers-Garry