how did Holland and Holland, Jeffery etc determine velocity of their rounds? What devices did they use?

By ballistic pendulum.

Shoot something with a known mass, measure how far it moves, and calculate the velocity from the movement.

Ahhhh.. Very good. Thanks

By Chonograph 1860 and onward + Crusher gauge 1860 and onward. Prior too that mathematical calculation validated by ballistic pendulum. The chrono proper and crusher validated the mathematical calculation.

Forgive my lack of creativity but what would an early chronograph have consisted of? I can only think of current models but can't think how that would have been accomplished with 1860 technology.

Screens, electromagnetic circuitry and the time it takes for a metal rod to drop when circuit is broken as the bullet impacts the screen.

Le boulenge chronograph of 1860.

Holland and Holland had their own testing range ? 1896 at Willesden later named Kensal Rise.

They assembled ammo on site, chronoed, did shooting at screens to measure actual trajectories as well as measure penetration on stacked wood boards.

Thanks ALF, I googled the Le boulenge chronograph of 1860 and there is a pic online. Interesting stuff...thanks again.