Though not being specialist in guns i can propose following improvements to Barry's approach :
a. As increase in voltage above 60V did not improve
projectile speed it is may be better to play with metallic ball and pulse width - to make pulse shorter.
I suppose the energy from discharging capacitor did not expire when projectile passed coil - that was resulted in back force to projectile and reduced further speed increase .
b.Good to put more than one coil and give a pulse sequentially to them to give gradular acceleration to projectile .
But above are improvements , so you have to go first for simple .
I think the page is full of details - so would be better to post questions on that design or to design you are goining to clone if needed.