Back in the "good ole days", how did they determine velocity? I'm not just talking reloaders either, but commerical loaders in the 19th century? Did they take the BC, bullet weight, and a drop chart and then use the curve of the trajectory to estimate the velocity?

In the real old days they used an hourglass.
Then they used a clock.

Alot of experimentation in velocity and remaining energy was done with a ballistic pendulum. You shot the pendulum at a known distance and bullet weight and measured the arc of the pendulum's swing. Crude yes, but it proved a number of theories involving air resistance and such before chronos were available.

Fjold, and from what I've read about the pendulums, they were quite accurate at their intended purpose.

I can imagine shooting a hanging block of wood as a chrono.
Some problems would be:
1) Each bullet makes the pendulum heavier.
2) If the block is hit off center, one gets some angular momentum.
3) If one tries to see how high the pendulum swings with a scale in the background, if one is not posittioned correctly, there is parralax error.

I made a ballistic pendulum out of a log and hung it by ropes under the basement stairs. I shot .22 LR's into it and measured the defection by letting it push a scale. The measurements were close to the 1100 fps that was expected.

One could do the math and add the weight of each bullet as the shooting went on. With my algebra ability and or having no calculator in the 1950's I passed over this refinement.

One evening, in an idle moment while teaching an evening physics lab, I heard shots coming from the lecture hall nearby. There was one of the profs, with an ancient .22, showing the class how to use a ballistic pendulum.

He had a pretty slick arrangement for measuring maximum deflection. The pendulum was a 4x4, about eight feet long, suspended by strings near each end. Underneath, he had a meter stick propped up, with a little cardboard slider. As the pendulum moved, it pushed the slider. Wherever the slider stopped, that was maximum deflection.

Even with this primitive setup, the class was able to determine that CCI Minimags were the most consistent and the most potent of the cartridges tested. But chronographs are a lot easier.

The pendulums were very accurate and they had a container on the top with bullets of the same size as being teste and
on every shot they remove one, keeping the weight the same.
When it was all shot up they hung a new, as it was neccessary to keep bullets in the pendulum after impact to get all energy translated to the movement of the pendulum,
and you could still stop it from swinging and reset scale
fast enough to do couple rounds a min...Ed.

I agree that they were accurate when measured correctly, the calculations and measurements were usually crude.