Help on Knockback weapons

On my new game king of the hill I am trying to make weapons that knockback those who are hit by the bullets like in Area 51 but it is not working.

for recoilforce i know,but knockback u see this when u create a zombie tag,have one script with strong projectile script

apply force on unit is the script need use.
apply force=10 its good on last touching unit
(use trigger when a unit starts touching in projectile)