The energy in the projectile is going to be related to the energy dumped into it. The energy in a cap is ½CV². The constant is the energy density of the package; the more farads, the fewer volts in a particular package.
In your case, the coil winding resistance has to be smaller because of the lower voltage. You might want to try something like a single turn. Wind the coil then cut it along the length of the launch tube. Then solder the pieces back together so you end up with a single turn made of multiple pieces of heavy gauge wire. That will allow the cap to dump its charge in the shortest possible time. The length of tube covered in windings and the size of the slug being accelerated will be maximized when the slug reaches the center of the coil as the current reaches zero at about 5 RLC TC. The handicap you are giving yourself by using a large size/low voltage cap is that the values of R and L have to be minute to get the cap to discharge quickly enough. You can, of course, compensate by making the launch tube longer but, then you are losing energy to friction.
By the way, be careful with that thing. With a cap that size, you are talking about something that could qualify as a rail gun. Tune it right and fire it in a vacuum or at least a low pressure box, I see no reason why you shouldn’t be able to get a projectile at least into the transonic range. In theory, you could get a small one into the hypersonic range.