before force is applied:
before releasing the shot: both are at reast
after releasing the shot: shot has velocity v, gun has velocity -u ( because in the opposite direction)
25xv + 5000x-u = 0
200u = v
now force is applied:
Impulse = change in momentum
10000x1.25 = 5000(u)
u = 2.5 ms-1
v = 200x2.5 = 500 ms-1