A large rectangular box falls vertically with downward acceleration a. A toy gun fixed at A and aimed at C fires a particle P. Then –

The effective acceleration due to gravity acting on the particle in the box =  $g-a.$ When

$a=g,$ the effective acceleration due to gravity on the particle becomes zero and the

particle moves along a straight line and will hit C. If  $a>g,$ the effective acceleration due 

to gravity on particle is negative, i.e. acting upwards, so the particle will hit the roof DC.
If $a<g,$  the effective acceleration due to gravity on particle is positive but less than g so the particle may hit wall BC.

If $a<g,$ and velocity is very small, then it can also land on side AB.