I suspect their intended use was for disabling things like cannon crews. Not necessarily for killing the individuals as shooting into ammunition piles with explosive bullets in the case of the largest calibers. My impression is that that was the purpose of the muzzle loading Jacobs rifles a few years earlier.
In terms of usefulness of caliber, remember that there were various types of swivel cannons and small artillery pieces of perhaps 1" or slightly bigger size. The Jingo guns and the earlier flintlock wall guns would have been a lighter and more portable version of the same. I can also see their use in shooting through wooden barricades to hit the people behind them.
cheers mooncoon