Here's how to get a secret weapon in Killzone 2 demo and blow some heads off

In the Killzone 2 demo you can do some serious head exploding damage to the enemy but only if you acquire the special gun. And here's just how you do that - throw a grenade at the guy that's standing on the roof of the building inside the warehouse. It will make him run and get blown up off the building and allow you to pick up his gun.