Bob,
The longer the better really.
I have an old flint Dragoon pistol barrel, 12" long, but never got it stocked up yet. (Richard Wilson)
We don't have turkeys up here, and I like my birding pieces if we did, but hoped to help Skychief make some decisions.
The above test was very basic, and left a lot of variations untried.
Might get at more trials soon.
I did try a load with a rolled paper shot-cup, base folded over on the wad, but the pattern was erratic. I should have tried a good few more, but fingers were freezing.
An old mate back in England used to shoot bolted rabbits with a double 20-bore pistol, so it can be done. I'd reckon a rabbit on the fly would take more stopping than a turkey standing there while you shoot his head.
Richard.