I tried it once with a reproduction flint lock. It didn't work. Seems the Labradar was maybe reading the velocity of unburnt particles of gunpowder? I only tried it for one shot, though, so maybe there is a trick to it, that I didn't know about.
In the video, just before the camera cuts away from the Labradar screen, you'll see the muzzle velocity is recorded as 3856fps, which is obviously too fast for a lead round ball out of a flintlock.
I have to set mine up for the pistol velocity range rather than rifle. If left in rifles it reads excessively high velocities when it reads. Once set to pistol it reads perfectly fine on my Knight Mountaineer 50cal with 495gr No Excuses. They are running around 1400fps.
My Paramount Pro 50cal is fast enough to stay in rifle configuration. It is pushing 330gr at 2200fps.