Both. The "ufs" are microfarads, which is a measurement of how much electrical energy can be stored in a capacitor. For examples, a 200μf capacitor can store double the energy of a 100μf capacitor. Volts determine the size of the magnetic field. For most small coilguns, 300-400 volts is a good range. Camera flash circuits are "okay" for charging these. Also, the amperage determines the strength of the field, the faster you dump the energy from the capacitor into the coil, the higher power your coils electromagnetic field will be. Now, if your capacitor is too large, and your switching circuit too slow, not only will the coil pull the projectile back, but the initial forward motion will be significantly reduced. You need a capacitor and switching circuit that will work fast enough to effectively drain all of the energy before the projectile passes the coil. In other words, you need a balance between voltage, capacity, and switching speed. P.S For a beginner coilgun, you shouldn't stray above a 2500μf capacitor bank, at 450v. For safeties sake.