nobody here understands Newton's Second law. This makes me sad for humanity.
edit: i'll break it down for you.
kinetic energy is mass * velocity. (.5mv^2 to be exact)
Velocity is the differential of acceleration to time.
Acceleration to an object in the vacuum of space is constant unless a force acts upon it (this would be gravity, thrusters, another object colliding etc). Unlike the previous post, velocity is not constant, acceleration is.
So the longer my projectile is in space the higher the velocity is. Since the velocity is going to increase at the same acceleration for as long as it doesn't collide with something, the kinetic energy will increase.
A projectile weapon like a cannon should have it's weakest damage at point blank range. It's highest damage at whatever the furthest distance the game will register an impact (4km seems to be the upper limit). While it may be unreasonable to ask the game engine to render correct kinematic physics like ever increasing velocity of projectiles, it would be trivial to modify the damage the same way (but inversely) as it's done for energy weapons.