[KenWittlief]

its not really force you are looking at - a bathroom scale measures force (the force that gravity is exherting on the mass you are weighing)

you need to measure the impulse that results from a collision - the energy contained in the projectile

another way would be to fire the projectile straight up, or to use a strobe flash on a camera and measure the velocity as the projectile passes.

or setup a light sensor light gate, with the projectile passing infront of two light sensors, spaced out a few feet apart