Mount the weapon so that the barrel is exactly one foot off of the ground and point it perfectly horizontally. Place a level on top of the barrel to fine-tune your aim.
Roll out the white paper on the ground as far as it will go along a parallel path to the barrel of the weapon. Inspect the paper for tears, and mark any that you find.
Fire the weapon. Because of the constant acceleration of gravity, the bullet will fall the one foot to the ground in exactly one quarter of a second. (See Reference 1)
Find the projectile's impact point in the white paper. It will be marked by a hole or tear.
Measure the distance from the impact point to the barrel in feet and multiply it by four. You now have your muzzle velocity in feet per second.