As much amperage as you can put into the projectile in the time you have that the projectile will reach the center of the coil.

So this meens as we get into higher speeds and velocities we have even a shorter amount of time to get as much kinetic energy into that projectile.

So for example lets have 2 different power source's:

Now you may be wondering why source #2's capacitance is so much lower that 1, and thats because we want to turn off the pulse as soon as the projectile reach's mid section of the coil so there is no suck back effect. So source 1 lets say will give you 1000 amps, and source 2 will give you 4000amps.

Now you see the problem right?

As we get into the higher voltages/amperages the projectile is moving much faster, and we dont want any suck back effect so we lessen the capacitance of the capacitors so that the pulse will be shorter. Even though we increased the voltages over 11x we only get about 1/4 the time to transfer the kinetic energy to the projectile and thus only 4x the amperage.