According to DARPA it was a .50 round:In reality the weapon used in the firing was a 12.7mm totally costom gun.
"Fitting EXACTO’s guidance capabilities into a small .50-caliber size is a major breakthrough and opens the door to what could be possible in future guided projectiles across all calibers.”"
Their guidance and control system is classified, so we can't really be sure if it requires a smoothbore platform. I was just suggesting that miniature smoothbores may be useful.
In any case, once this tech is worked out, only then will we max out the ultimate potential of gunpowder propelled bullets.