i want to use arduino to trigger my coilguns second stage, but i dont know how. any ideas?
Can you post a schematic of your second stage? Is it a fixed time interval or do you have some kind of sensors for firing based on projectile position?
Most likely you are needing to switch a high current on for a short period of time. A MOSFET drive circuit, similar to that of switching a solenoid on and off, is a likely candidate, but much depends upon the details of your coil gun.
--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons
what is a coilgun? Please explain with a little detail
A very cool toy 
http://wiki.4hv.org/index.php/Coil_gun
--
The Quick Shield: breakout all 28 pins to quick-connect terminals
it can be called MAC.
Magnetic Acceleration cannon.
Use a magnetic field to accelerate projectiles. (objects)