Benjamin Robins measured velocities with a ballistic pendulum - allowing for the probable (small) errors in construction of the suspension system, accurate weighing of the pendulum and measurement of the movement when struck by the projectile, and considering that his mathematical calculations, while painstaking and slow, were correct, his calculations of velocities are quite accurate.
Robins was one of the first scientific ballisticians, and his work is (rightly) highly regarded.
mhb - Mike