Rigged weapons animations and humanoid avatar

Hi,

I'm pretty new to animations, I'm using blender for it. I have watched many videos on youtube, but couldn't find answer.

I've created humanoid in blender with animations, I'd like to add to my model some guns. If I add weapon to my character armature then it fails in Unity (gun at wrong position and transformation errors). How should I start adding guns?

For now, I'm using separate models and attach gun to the hand bone, but I have no idea how to add more complex animations like reloading with separate models... it would be really hard with IK.

I want to create FPS game, but not just arms with gun, but whole body so I can add multiplayer later. Is there are any good tutorials you know about it, please link it.