A plain smoothbore, without anything but a basic loading of powder, wad, shot and overshot card, will shoot like a cylinder bore shotgun..., wider than IC choke, and if you shoot birdshot from a plain jane slug barrel you will see the same results from a modern shotgun.
The length of the barrel will have very little to do with the pattern..., it may have a negative effect on the impact of the shot if the barrel is very short, say under 24".
For loading a smoothbore you start with the same volume of shot with the powder. So IF you use a 70 grain measure, you should use the same measure for the lead shot..., mine gives me 3/4 of an ounce to 7/8 of an ounce depending on the size of the shot. That's roughly the same as a plain, over the counter, upland bird load. Sometimes I load a bit more shot so I'd use 80 grains of powder, and a measure set to 100 grains for the shot...gives me a tiny bit more than 1 ounce of shot.
There are various things you can do..., thick paper cartridges, use of modern shot shell wads, shot loads held together with wax, jug choking, modern choking, and installing modern choke tubes...to change the pattern.
I have lost the reference but I remember I had images of two old woodcuts, both were of hunters, with German names, and one was Jaeger, but the other was very different and showed a fellow with a gun and several birds plus his dog..., it showed that standard hunting and bird hunting were thought at the time to be very different... bird hunting can deal with a complex bit of variables...
LD
