The motion of the weapon is a part of the animation. The animation is from the free animation pack from the market. I mention in a previous video that this is not something I will make in this project.
This system is actually using the muzzle of the weapon itself as the point of origin for the bullet. That is why the crosshair is moving the way it is. It is tied to the animation. And to reiterate, the animation is something for the buyers of this project to sort out for themselves.
You can make a fixed weapon socket instead and not tie it to the characters animation. It will take care of the erratic movement of the weapon.
Also, there is no recoil either. This is also a part of the animation.
The error is an error related to the network checks I believe. I am aware it is there, and it is being worked on. I believe it might be a bug since it is working as intended even though I get said error, but I am looking into it. It won’t be available in the marketplace before Epic says it is ready, and I doubt they will say it is ready with errors like that.
Btw, you were that guy 
and it is good, someone has to be.