One can find the velocity of a projectile with a pendulum. Shoot a rock at a pendulum. The pendulum must have a known mass distribution.  The projectile must NOT bounce. In other words, the projectile must “stick” to the pendulum. 

The physics principles that are applicable are:

Momentum = V*mass.  Not V*weight.  This is a common mistake.

Momentum is always conserved (as opposed to kinetic energy which usually isn’t conserved.)

Momentum of the projectile before impact with the pendulum must equal the momentum of the pendulum-projectile combo at the moment after impact.

Upon impact, the pendulum will start to swing.  The velocity of the pendulum-projectile combo right after impact gives rise to a quantity called kinetic energy = (1/2) *m*v^2 (where m = mass of the pendulum-projectile combo, and v = velocity of the same combo).

As the pendulum-projectile combo swings, it is displaced from the equilibrium position.  This will cause the center of gravity of the pendulum to be raised.  The ascent of the center of gravity gives rise to a quantity that in physics is called potential energy.  Potential energy = m*g*h.

The potential energy of the pendulum-projectile combo at the apex of the swing = to the kinetic energy of the pendulum-projectile combo at the start of the swing.

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You can adjust for friction (air and bearing) by letting the motion of the pendulum slowly come to a stop.
Then extrapolate back up to the initial velocity at impact.

Here is an item of interest for experimenters. Also search under “Swing Speed Radar”.
I’m still working on my ballistic pendulum just for fun.

http://www.gloveradar.com/