Longer demonstration
Simply enable the "All weapons" codes and kill the fire creature. This will delay the sequence and keep the demo from ending.

Hidden ammunition
When you first start your game, go into the elevator. When the doors open, go into the room on your left. You will find a crowbar. Hit the soft drink machine and get out of the way before it falls on you. After it falls, you will see some gas spraying out. Now jump over the machine and crouch down. Then go over to the little hole in the wall. Then press E to shut off the valve and the gas will stop spraying. Now go in the hole, turn on your flashlight, and climb up the ladder. Keep on going and the floor will fall. You will come to a secret room with shotgun and pistol ammunition.