If you ignore the code section at the beginning of the C++ Shooter tutorial, there is an Blueprint animation explanation at the bottom:
You may want to look into that C++ tutorial as an idea for how you would add a weapon with Blueprints as well.
Here is a video on adding a custom weapon to Shootergame (No code involved) as an example of how you would add your own to a new Character BP:
Also, there is already a discussion about custom weapons going on in thread: [REQUEST] FPS Weapon System in Blueprints - Programming & Scripting - Epic Developer Community Forums
Let me know if any of helps or if you have any specific questions.