Best Way To Animate Hands

So my name is Tyler and while I've been coding and making little passion projects for years, I've recently become much more dedicated to learning game development. My issue is that I'm not exactly sure how to go about rigging fps hands and making the animations interchangeable with whatever item I decide to place in them. I started by making simple animations with no hands but whenever I apply a different model to the item (weapon)s transform is all wonky and wont perform the way I'd like. How do I go about making the new weapon model take the transform of the animation properly?

 

@TaintedJester
Do you have a short vid of this problem?