The smoothbore muskets used during the Revolutionary War actually did shoot unpatched roundballs. Soldiers tore open a paper cartridge holding powder and a ball and first used a small amount of that powder to prime the pan, then poured the rest of the powder down the bore followed by the ball and then used the paper from the empty cartidge as an over the ball wad to keep it from rolling out.
Yes, they were drilled to prime before loading, it was war afterall. This is one place where re-enactors take liberties from authenticity in the name of safety.
I have experimented with unpatched .457 roundballs loaded into a .45 caliber TC Hawken and it actually worked fairly well. A tap from a mallet gets the oversized ball into the muzzle and from there on, it's not hard at all to push down bore to the powder charge. You do have to wipe the bore with a cleaning patch between shots though because without a patch, the ball does not "clean as you load".
A bullet does not have to have a hollow base for the chamber pressure to upset the bullet and make it fill the rifling grooves as is proven by TC MaxiBalls. Roundballs upset to obturate the bore also.