The beauty of a blunderbuss is that most were fairly large bores, even considering the times.
Wool blanket wads, a flask of powder, and just about any kind of projectile one could imagine, or get their hands on.
IMO, it's muzzleloading at about its simplest. The larger the bore size, the greater the availability of projectiles that can be utilized.
A single, bore sized lead ball. Small pieces of scrap iron. Small stone pebbles, such as Hungry Horse has mentioned. Lead buckshot, in varying sizes. Birdshot. Wooden or iron darts, such as a modern fletchette round mimics. The list goes on and on.
I don't think there could be many wrong answers. If one is trying to create a persona, then a time frame, and a country of origin would be necessary. Otherwise, just do as you like, as long as the powder charge size, and projectile(s) stay within safe limits for the barrel/breechplug combo.
What is an effective range? The bore size, the powder charge size, the weight of the projectile(s), combined with the amount of flare in the bell, probably has a lot to do with maximum effective range.
If I had one, I would treat it as if my life depended upon the effectiveness of the blunderbuss. I'd go to a large appliance store to acquire as many large pieces of cardboard as I could obtain. I'd make myself a large patterning frame. Buy a large roll of brown wrapping paper. Buy a box of Sharpies. Make up.a bunch of wool wads. Purchase enough Fg powder. And start shooting. A LOT!!!!
Unless you only want a blunderbuss as a novelty gun, and that seems to be how most owners treat theirs, then A LOT OF SHOOTING is the only way to figure out a buss's capabilities. Lots of rounds down range, with as many different types of projectiles as you can imagine.
If you think what a sawed off double barreled shotgun is good for, then a blunderbuss is most likely in the same category. 10 yards/meters and closer.