How to mod in a new custom gun?

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Hi! I'm currently trying to learn how to make a mod, which will either replace an already existing gun, or add in a new one entirely (preferably a new one entirely).
I have no idea how to go about this, but I really want this mod to be a reality so I'm looking for any way to learn.
I want to add this model into the game: https://www.artstation.com/artwork/8wGwER

Its rigged and textured, all I'd need is to animate it and then add into the game as far as I'm aware, though I'm sure there's more to it.
If anyone could help me by pointing me in the right direction, or giving me advice I'd appreciate it a lot!

Thanks!
 
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