Do you mean "bare ball"? I think velocity with a dry ball is....wait for it....0fps. ? I don't know, but I'd like to know. Never thunk of that before. With good wads/wadding under the ball, and with a reasonably good fit to the bore, I bet it wouldn't be anything drastic. The proverbial "good question".
I fire bare-ball in my .58 TC smooth bore trapper, the "Little Beast". Using a .570" ball in it's .580" barrel, if I chew the ball it's a really nice fit. I put a waxed wool wad, and then a plain wool wad under the ball, I've been assuming that should seal the bore reasonably well. Although, I'm sure there is some blow by with bare ball. How much?
If I got really bored this summer, would not be that hard to send some bare balls, and patched balls over the chrono with Bess I guess. ?? Someone must have done that.