..use a series of coils, each with more windings than the last and pulse them in sequence. You might want to use something like an IR sensor preceding each coil in order to see where the projectile is and to fire the coil as it approaches. The last coil should probably have a sensor which deactivates the coil as the projectile just enters it - leaving it enough time to impart some velocity but not leaving it on long enough to actually cause drag on the projectile as it leaves.