Putting a resistor in circuit will increase the response time, but that will reduce the force acting on the projectile in the beginning. You would get the same effect by reducing the charge voltage on the caps. What you, err... OP, ideally wants is maximum current going through the coil in 0 amount of time (maximum magnetic force) but turn it off right when the projectile gets to the center, otherwise the magnetic force will be pulling the projectile back to center after it gets to the other side, slowing it down. Since we have to deal with this pesky real world physics, there's a rise/fall time in current change that is dragged out due to resistance, so you ultimately have to switch off the coil before the projectile gets to center. There are a few ways to go about this, which is why engineering is so fun!