A ball of mass 0.25 kg is fired with velocity 100 m/s into the barrel of a spring gun of mass 1.6 kg initially

A ball of mass 0.25 kg is fired with velocity 100 m/s into the barrel of a spring gun of mass 1.6 kg initially at rest on a frictionless surface. The ball sticks in the barrel at the point of maximum compression of the spring. No energy is lost to friction. What fraction of the ball's initial kinetic energy is stored in the spring?

because the question says that the gun is initially at rest, i am assuming that it will move after the collision. So i used conservation of momentum to find the velocity of the gun+ ball after the collision, to calculate the kinetic energy of the complex. Then using conservation of energy i subtracted the kinetic energy of the complex from the original energy of the ball at 100 m/s and got 1081.08 as a rounded stored energy, so the fraction (rounded) would be 1081.08/1250 (1250) is the kinetic energy of the ball at 100m/s)
hope im in the right ball park