Engaging the rifling
The most salient point yet is that the round ball is going to have to engage the rifling to test the theory, therefore it would deform the roundball.
I was thinking a round ball made of depleted uranium, and lands and grooves machined out of titanium or even diamond edged lands ...
And then I saw that really fine flintlock and thought about all those steaks, and stews and burgers and backstrap ... and raided the refrigerator!
Just a giggle that comes to mind. We print out targets we download from friends over the internet ... at which we shoot smoothbore flintlocks ... that tickles me to no end.