If you are wanting a shotgun pattern, here's how to do it. (For a .625 Big Bore) You need BBs, 18. ga. cloth patches, and some 18.ga muzzle loading cardboard wads. (Or you can trim 12 ga. patches and wads to size). You'll also need something long enough for a ramrod. Start by loading one of the wads. Next, measure out 15 or so BBs, and pour them down the barrel. Place another over-shot wad on top of the BBs, using the ramrod. Next, place the cloth patch over the barrel, and ease it all the way to the BBs with the ram rod. Push it in snugly so you get a good, tight air seal.
Now, when your ready, just fire it as normal. You'll get a decent pattern out to around 10-15 yards. You can adjust the pattern by how many BBs you load. Less = tighter pattern. More = wider pattern. I hunt quail like this all the time (during the season, of course...). Quail usually fly up right under your feet, so by the time you get your blowgun up and aimed, it's only about a 5-7 yard shot. It takes a little practice, but it's loads of fun (and feeds you as well..).
For a .40 cal, just use .410 patches and wads, and trim them slightly to fit. For a .50 cal, use 28 ga patches and wads, and trim them slightly to fit.