Rifled slugs vs unrifled have been a subject of debate for years. Accuracy tests go both ways between rifled and un-rifled slugs in shotguns. The way they are commercially made (in a swage press), adding rifling is no big deal anyway, and it helps the marketing departments out when they are advertising.
In a small cannon, whcih probably won't have a tight fitting bore, the rifling probably can't do anything usefull anyway.
That being said, unrifled slugs (which fly the same as badmitton shuttlecocks) are real easy to make. I have made them for my 12 G shotgun using a Lyman mold. You can cast at least one a minute with a single mold. The resulting slug is going to weigh less than a solid ball, making recoil less of a factor.
Since you arn't passing the slug thru a choke, the bullet metal doesn't have to be soft and can be anything you can find for cheap or free.
