Hello and thank you, Scel! So Oneirality is developed in Godot 3, but the bouncing bullet shells can work in Godot 4. 

Essentially, each bullet casing is a KinematicBody2D (CharacterBody2D for G4) so that it makes use of that node's physics collisions. The casings run a function in the _physics_process that will move the casing downward if its global_position.y value is not less than bound, otherwise it will multiply its velocity by -0.75 so that it bounces up and multiplies velocity.x by 0.5 so it slows down horizontally. It also runs a check to see if its velocity is below a certain threshold to tell it to set_physics_process to false. There are other functions used, but for the main bouncing, this is it. I'll paste some of the code below:

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export (float) var bound = 0.0
export (int) var bound_rand = 4
export (int) var motion_rand = 150
export (float) var bound_stop = 0.75

var motion: Vector2 = Vector2(0, -200)  # When instanced, the casing will move upward already. For G4, you can put this in the _ready function.

func _instantiate():
    if not bound_rand == 0:
        bound += (randi() % bound_rand + (bound_rand / 2))
    motion.x += (randi() % motion_rand - (motion_rand / 2))
    motion.y += (randi() % (motion_rand / 2))

func apply_bound(_delta):
    # Should Fall
    if global_position.y < bound:
        motion = motion.move_toward(Vector2(motion.x, motion.y + 10.0), 10.0)
    # Should bounce
    else:
        motion.y = -0.75 * motion.y
        global_position.y -= 0.1
        motion.x = 0.5 * motion.x
        emit_signal("bound_contact")
    
    # Stop physics to prevent performance issues
    if motion.length() < bound_stop:
        set_physics_process(false)
        _bounce_stop()  # This just rounds the casings global_position to be pixel perfect
        emit_signal("bounce_stop")
    
    motion = move_and_slide(motion)