A gun of mass 400kg, fires a bullet of mass 100g, with a velocity of 200 m/sec?

M1=400. U1=0. V1=?
M2=0.1. U2=0. V2=200
Now
M1*u1+m2*U2=m1*V1+ M2*V2
0= 400*V1+0.1*200
V1= -20/400
V1= 0.05 m/'s
Negative sign describe backward movement of rifle