I have shot round ball wadded with tow a fair amount from my 20 ga. flintlock smoothbore. When I first started working on those loads I simply loaded power-tow-ball-tow, and had pretty good results. I noticed the loads tended to shoot a bit lower than loads with modern wads, and that the recoil was not as heavy. Tow is pretty porous stuff, so I decided I was getting some gas blowing through it and it wasn't giving me a good seal. I tried loading one 1/32" overshot card over the powder before the first tow, and could see a definite improvement. Now I always load some kind of an overpowder wad, frequently just a folded piece of brown paper, and my results are good. I do this for both ball and shot. My overpowder tow wad is big enough that it makes a 1/2" thick wad when well rammed.
I shoot .600" self-cast balls and 85 grains 3F Goex.
Twenty-five yard 3-shot groups of less than an inch from a casual rest are typical for me with tow.
Spence