Reverse polarity using MOSFETs?

I would like to make a self-reloading, railgun-based device, that moves an aluminum rod forward at high velocity (typical railgun) and then backwards with low velocity. This would be controlled based on Arduino, naturally. An example of what I want the code to look like (not actually code, just the general idea):
If Button 2 pressed:
digitalWrite(A1, HIGH); //sends 'on' signal to MOSFET
delay(500);
((something to reverse the polarity of the circuit so the launcher moves backward slowly))
delay(2000);
//end
Thing is, I have no idea how to reverse the polarity of such a circuit using MOSFETs. Anyone have any ideas? I think the velocity of the projectile is based on the voltage/current applied across the rails, so I could use two power sources, one 9V and one 1.5V for example. (Depends on how fast I need it to go - I can also do 150V from a boost converter, if that's even necessary). I would also use a capacitor bank for the forward launch.

Just asking: Have you looked to see what is involved in actually building a working rail gun?

You can reverse voltage with a bridge circuit, which requires 4 MOSFETs or 4 transistors. Or, you can use a relay (IIRC a DPDT relay will do it)..

so I could use two power sources, one 9V and one 1.5V for example. (Depends on how fast I need it to go - I can also do 150V from a boost converter, if that's even necessary)

A boost converter doesn't increase the amount of energy. i.e. If you boost the voltage, you get less current.

I would also use a capacitor bank for the forward launch

A capacitor does store energy and it might help to give you a very-short burst of high-energy. But, they don't store a lot of energy (depending on the capacitance & voltage) so for example, you can't power a flashlight with a capacitor (at least not long enough to make it useful).

I thought if you boosted the voltage, by Ohm's Law, you'd get the same amount of current for a shorter amount of time.
And yes, I have researched rail guns before.
I don't intend to power the railgun solely off the capacitor, of course.
Can you link me to a schematic for an appropriate H-bridge circuit? I get the general idea, but haven't managed to find a specific outline.

There are no hobby-type H-bridges that work for the currents and voltages typically present in a operating rail gun.

Does the launch tube use a single magnetic coil, or a series of coils activated in sequence? The latter could be one way to control velocity without having to resort to different voltages, or even polarity.

jremington:
There are no hobby-type H-bridges that work for the currents and voltages typically present in a operating rail gun.

Mine won't be running at the standard level of power of a typical railgun - I want to launch a projectile, not smash it to bits. More powerful the better, but if there are limitations, so be it.

Another question I have: Say I lead a digital pin out to one lead of a button, and the other lead is connected to a MOSFET. Would it be possible to write a code that waits until the button is pressed, then sends a 5V signal to the MOSFET through the button? I'm asking because I ran out of usable pins on my Arduino, and am trying some unorthodox stuff in order to fit everything in.