That is a pretty cool coilgun. But, using 8 AA batteries I wonder how many times it'll actually fire before the batteries go dead?
Lets see if I remember all the correct conversion factors...
The capacitor pack is 1000V at 600uF. Total power in the cap's is (1/2)cv<sup>2</sup>=300 Joules
A single brand new AA battery is about 2000 milliamp*hour at 1.5V, which is 3 watt*hours which is 10.8KJoules. There are 8 batteries so the total energy in them is about 86kJ.
So, the battery pack will fire the gun about 86KJ/0.3KJ = 290 shots. (YMMV, and I'm pretty surprised that it should fire that many times on one set of batteries!)
How fast might it fire a projectile? (Note that the linked page doesn't quote muzzle velocities.)
Figure a coil gun is 1% efficient, that means the projectile will have a muzzle energy of about 3J. Assuming a BB as the projectile (mass= 0.34g) and using Energy=(1/2)mv<sup>2</sup> we calculate that a BB might be launched at 133m/s (436 fps). Not too bad, but a $40 BB gun will out perform the coil gun.
Coil guns have huge bling factor but they are really crappy ways to get a projectile up to high velocities.
EDIT#1: speeling and engrish