>>1346932>>1346931to explain further, the current flows from behind the projectile, through it, and then back around again to complete the circuit
with the right hand grip rule (thumb = current, fingers curled = magnetic field), you see that the magnetic field always curls upward between the rails, behind the projectile
then you use the fleming's left hand rule™ in the space behind the projectile (index finger = magnetic field, palm = current, force = thumb), the projectile is pushed away from the power source