It would be a similar principle to the one they used to make the paintball gun shown on one of the Christmas Lectures. A paintball gun uses compressed air to fire the paint pellet and is, in effect, a type of blowpipe.
Could this be something like a rail run? This is where you have a metal projectile, and an array of electromagnets along the barrel. You switch each electromagnet on in turn, which then accelerates the projectile along the barrel and causes it to shoot out of the end.