Why do coil guns contain capacitors?
Each battery cell has an internal series resistance and when you stack up the batteries, these series resistances add up, too, limiting the current you can draw. You can load a capacitator with that limited current. The time it takes is not relevant for the function of your coil gun.

When you trigger the coil gun, the capacitator is discharged very quickly at a very high current. It can supply this current because its own series resistance is much lower than that of the battery stack. The height of the current peak is practically only limited by the inductance of the coil you use.