The lessons are meant to teach you how to build the weapon, the project files are there to help you. Like I mentioned before the grenade was only used in one lesson and never shown again and subsequent lessons continue with the main weapon mechanics.
You can use that lesson to learn how to shoot other projectiles as well. The bullet ejection lesson also teaches ejecting projectiles from the side of the weapon.
Lesson 9 (scene 9) continues on with a weapon that was shown in Lesson 7 as a starting point. Each lesson starts off where the last lesson ended, so the scenes are made to reflect that. If you want to see the completed weapon then you’ll find it in scene 11 of the project files.
