Firearms of the Confederacy by Fuller and Steuart includes figures for U.S. arms sent to Southern arsenals during 1860. These were urgently needed to arm militia in case of another attack. The book may also include something about rifling of older smoothbores. If I recall, the twist was made for three turns. Page 38 of the Jack Coggins classic, Arms and Equipment of the Civil War, provides a comparison of effectiveness for the smoothbore and rifled smoothbores, both firing a round ball.

Of course certain weapons like the Mississippi rifle were designed and made to fire a round ball. You're asking about smoothbores which were rifled afterwards.