Shockologist

Hello everyone. I am building a coil gun and I have come to a dilemma, which brings me here. For the most part every coil gun I have seen uses a projectile similar to a bullet and a barrel. Why has no one used a projectile thats a foil shaped ring riding on the outside of the barrel instead of a bullet on the inside. I'm considering testing this on a small scale to see if it travels farther/faster. I believe that a ring that rides directly over the coils would have a better magnetic attraction. Which leads me to a few questions; is there a greater magnetic field on the inside of a coil opposed to the outside of a coil? Do you believe a O shaped bullet would fly well, or would it tumble and be inaccurate? Is this a good idea or a waste of metal? Any comments, ideas, ect are welcome, thanks !