Create a new scene with Area3D as root

Rename it to Pickup

Add a MeshInstance3D as child

Rename it to AmmoMesh

Add a CollisionShape3D as child of Pickup

Select the AmmoMesh Node

Set the Mesh property to a New BoxMesh

Expand the Mesh

Set the size to 0.5m on all axes

Select the CollisionShape3D Node

Set the Shape property to new SphereShape

Expand the Shape

Increase Radius to 1.0m

Attach a Script to the Pickup node

Add export variables for 

ammo type and amount

Connect the body entered signal

Of the Pickup node to the script

To give the Player access to the ammo handler

Return to the Player Scene

Select the AmmoHandler node

Right-Click it and choose

% Access as Unique Name

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NEXT: Pickup Shaders