One of the comments left at the site states that one person tried using BBs but they kept welding themselves to the rails. Not sure if ball bearings would be much better. Stainless steel or chrome-plated ones, perhaps.
Here’s another 400v railgun project that explains a way to get around the welding problem, even if the projectiles are prone to it:
http://www.instructables.com/id/Rail-Gun-Linear-Accelerator/step4/Pneumatics/
“The metal projectile cannot simply be inserted into the rails, as you’ll get electrocuted, and the projectile will get spot welded onto the the rails by the large electrical charge suddenly jumping across the projectile.
To prevent both of these things happening, the projectile must already be moving befor it meets the rails at a sufficient speed to prevent it stopping. My system used 4 small gas reservoirs (you can use one, but these were all i had at the time, and my other large cylinder was attached to another project). I charged mine to around 120PSI, but the amount of pressure you need depends on the weight of your projectile. I didn’t need such a high pressure, but it was more fun this way.
You don’t have to used compressed air, but this was the quickest and easyest way for me as I only had to put the components togeather instead of build any parts.”
So, just get the projectile moving a bit before it hits the rails and you should be okay. I bet a spring mechanism would work as well as compressed air.