Question

a bullet has a very small inertia due to its small mass. but why does its impact its so strong when it is fired from the gun ?

Answer 1

Due to high momentum which it carries with itself.

Answer 2

The term "inertia" is a property of mass while the linear momentum is directly proportional to the mass and the velocity of the moving body. Since the velocity of the fired bullet is greater, therefore its momentum is also greater. Thus, the bullet impacts strongly when it hits a target.