Try steel ball bearings for projectiles, that is what I see used most often with railgun projects.
The real issue that you will run into is that you need a whole lot of power to hurl that projectile with enough force to do anything significant. Chemical propellants simply outclass railguns when it comes to efficiency.
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.