Splitting an Rigged Character

Hello everyone. I am making a first-person shooter in unity. When the player moves their mouse on the y-axis rotation (up and down), I want the player to move their arms and weapon up as well, so it appears that the player always has a gun with them and the reticule does not have to change. I asked many people on what is the best way to do this, and most responded with a solution of opening the character in Blender and dividing the arms and body/head and then importing them back into unity. I tried this, and although I did not know how to delete specific bones for each object (such as deleting all bones of the head and body) as the standard method didn’t work as this was downloaded from a 3rd party source (originally fbx. most likely not meant for blender) I later deleted these bones in the unity hierarchy after importation to avoid overlapping of bones. Even after this none of the original animations worked with my new model of split arms and body, and even after experimenting with only the body/arms when I 100% know there will be no bone overlapping the animations still do not work. Does anybody have any solutions to this problem, any help will be highly appreciated.

Essentially by “splitting” the rig you are creating a different hierarchy/character. The pre-existing animations will not work on the “new” rig.
Solution - after getting the rig conformed to your needs, create animations for it.
Using the generic setup animations can be created externally OR internally (in Unity).

To simplify - only fps arms/weapons are needed for a fps game. What is the purpose of head/body? Do they somehow add to gameplay?