Splitting a Bullet - Godot Tutorial 

It is easy and simple.You just have to make the bullet have in its own group.Use an area node in the asteroid script to detect if the bullet(using the body and group name) has hit it.if this does then use the same functions used in the video to make it split into two pieces.(I haven't watched the video yet.so I can't tell if the process might be that easy or not.but good luck understanding the code,the works will be the same) And you will be able to split the asteroid into many pieces as you want after you have done it.(But a quick reminder don't fall into any kind of rapid hole .if you find it hard to complete it within 2-3 days then stop developing games for a while.you will simply find the answer to the solution by interecting with the outside world. But don't get into fixing it immediately as you find the answer cuz you may lose courage to dev if you fail again.so only with enough gathering information and courage get into It

You probably could actually do that. However there are some cases where it's good to know how vectors work and know what you can do with them through mathematics. This tutorial is more of to teach how vectors work. Also in the project this was originally made for I'm using pixel art and I don't want to rotate the images but rather use the velocity to decide what sprite to show to the user.

A shotgun would have to work a bit differently and I've never built one, but feel free to ask for help on the discord and I'll see if I can help you build one