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Best voltage for coil gun?

In my coil gun project I have 36 330v 120 uf flash capacitors. When I had them arranged in 4 banks of 9 linked together in series (.5*1200*1200*1000e-6 = 720J) the projectile could easily punch through cardboard. But when I tried to increase the power by making 7 banks of 5 giving me (.5*2100*2100*500e-6 = 1100J) the projectile has trouble getting through cardboard. The coil I am using is 50 feet of 22 gauge wire. Is there any thing that I am missing? Thanks.