Question

A bullet fired from a gun on striking a target becomes hot why

Answer 1

Because of Large intensity of energy in form of heat. ..

Answer 2

All actual collisions (bullets hitting targets are a class of these collisions) result in a loss of kinetic energy. The kinetic energy of the bullet is not lost, it is changed from energy of motion (kinetic energy of the bullet) to heat.

Most of that energy is transferred to within the bullet itself in the form of heat. This heat is generated by the bullet’s internal friction caused by the deformation of the bullet upon hitting the target. If you have a chance, and can do it safely, touch a flattened bullet shortly after it strikes a metal plate target - it is quite hot.