Well mainly because the power supply i have is a kit I built a few years ago, and only goes up to 12 volts. But the main reason is i want to be able to finish the coil gun and know that i made it from "scratch."
That, and the fact that i need to drag my presentation out to a full 40 minutes, so I'd rather go through the formulas, and show how the boost converter works rather then just say "well this power supply charges the caps". But yes, using a power supply would be MUCH easier, but just ask my mentor, "easy" is not in my vocabulary

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Whats the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons, civil engineers build targets