How will a bullet having very high velocity and having little mass behave when fired from the Earth perpendicular to the surface? It will be a great help if somwone explained it? Will it come back? Or will it escape into the outer space?
Yes, it will come back with a speed high enough to kill people. Firing shots in the air is a stupid as urinating against the wind but far more dangerous and damaging.
If you fired the bullet greater than the escape velocity (more acurately greater the escape-velocity/√2), the bullet will not return in the earth. For earth, the vallue of escape velocity is 11.2 km/sec. Because at a higher velocity, the bullet will get a huge kinetic energy and so that the bullet will easily overcome gravitational force of the earth. Note: Normally a bullet can not achieve this high velocity. So, it will hit on the ground.