Some period muzzle velocities are available from various sources. Jack Melton has compiled some of this info on his website. http://www.civilwarartillery.com/ I think he only has listed smoothbore velocity data at the bottom of the technical information/range tables page.

The air drag is considerably higher for the smoothbores vs. rifled pieces due to greater cross sectional area, less aerodynamic shape, and lower sectional density. I've been wanting to work out some range tables based on initial velocities as well.

My interest is in the differences in trajectory and velocity at 800 yards and greater. That is where I expect the rifled vs. smoothbore velocities to cross for field artillery.

Some of the published contemporary time of flight values appear dubious in certain instances. Part of that is that these might be set for setting fuzes for aerial burst of case ~75 yards before and some feet above the target. I suspect that the methodology would not have been consistent.