Three K’Nex weapons with slingshot firing mechanisms, left to right:

heavy cannon; Springfield riffle; handheld crossbow

Instructions found on Instructables

projectiles for all three weapons

Firing the handheld crossbow. Notice the bullet above the window on the right side of the house.

Firing the heavy cannon.

Notice the bullet at  the far right-hand side of the photo in front of the basil.

You can tell that’s basil, right?

Firing the Springfield riffle. Notice the recoil on the rubber bands.