Conservation of momentum: m1v1 + m2v2 = 0
Let subscript 1 represent the shot mass, and subscript 2 the gun. m is mass, v is velocity
100*400 + 1000*v2 = 0 so v2 = -40 m/s (I assume you meant m/s for velocity, not ms !!)
Note that the negative sign indicates the gun travels in the opposite direction to the shot.
Assuming constant acceleration (deceleration here) of the gun we can use the constant linear acceleration equation
v2 = u2 + 2as to find the acceleration, a:
0 = (-40)2 + 2a*(-0.12) so a = 1600/0.24 m/s2 (Note: I'm assuming direction is positive in the direction of the shot)
Newton's second law says force = mass*acceleration, so F = 1000*1600/0.24 N