A way to measure force...

[KenWittlief]

one way to measure the energy of a projectile is to fire it into something like a lump of clay, suspended on a cord, and measure how high the mass swings up after the impact.

[Alan Anderson]

I think what you want to measure is either kinetic energy or momentum, not force. The force exerted on a target by a projectile depends on how long it takes to transfer the momentum -- the longer the time, the smaller the force.

Ken's suggestion of capturing the projectile with a pendulum and noting how high it swings sounds like a good one. The height will be directly related to the kinetic energy carried by the projectile.