The theory of impulse-momentum states that "the change in momentum of an object equals the impulse applied to it" meaning that the long barrel of the rifle covers a huge amount of motion of the bullet. The shot of the barrel is more accurate that way.
While work-energy theorem which states that "the work done by all forces acting on a particle equals the change in the particle's kinetic energy" meaning that the discharge of the bullet caused by the barrel is slower but has more power than the one with the short barrel.
