I think I am the first one to figure out this.

To fix the guns position in the back from any armor you like, you can do the following:

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OPTION1: USE AN ALTERNATIVE ANIMATION SET FOR THE TORSO
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Open the corresponding .upk file in the editor, and open the corresponding SkeletalMesh. The problem is related with the bones SwordSheath, GrenadeLauncherSheath and PsiAmpSheat.

You cannot edit the position of the bones inside the editor, and if you try to change it in a modeler like Blender for example, you will not solve the problem (I tried). The solution in this case is much simpler. If you go to the animation tab you will see there is a set of related animations in the game that affect the SwordSheath bone. Every animation uses a slightly different position. To apply one of this positions, open in the browser of the editor the corresponding content file and add the animation in "Unit Pawn Anim Sets".

From more to less distance to the body, the animations are:

Animations:
**************
AnimSet'SldCnvMed_Std.Anims.AS_SldCnv_Med_Std_Torso_A_M'
AnimSet'SldPwrLgt_Std.Anims.AS_SldPwr_Lgt_Std_Torso_A_M'
AnimSet'SldCnvMed_Std.Anims.AS_SldCnv_Med_Std_Torso_A_F'
AnimSet'SldPwrLgt_Std.Anims.AS_SldPwr_Lgt_Std_Torso_A_F'

However, these animations included in the Vanilla game only change the position of the sword.

With the DLC Anarchy's Children, Firaxis introduced lighter armors, and they added new animations modifying the positions of all the Guns at once. These animations cannot be accessed by default, but they can be located in the PATH ‘XComGame\CookedPCConsole’ in the file ‘DLC_30_SldCnv_Std.upk’ if you own the DLC Anarchy's Children.

You need to copy them into your mod, and reference your copies, to avoid broken links. If you don’t own the DLC, You can find the animations in this mod .upk file. In any case you should look for the assets called:

Animations:
**************
AS_Torso_A
AS_Torso_A_F
AS_Torso_B
AS_Torso_B_F

Put only one of these animations in the "Unit Pawn Anim Sets", as they are alternative versions with different modifications to the guns positions.



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OPTION2: SHORT TUTORIAL TO MODIFY ANIMATIONS WITH 3DMAX
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To fine tune the positions of the Guns, the corresponding animsets can be exported to 3DMax, then, the corresponding bones of the mesh can be modified, and the assets imported back. There is some tricks here that should be taken into account, but is nothing that cannot be solved with a pair of trials. Then main steps are:


1.- Export from XCOM2 Editor
-----------------------------------------
- Open animation file in XCOM2 Editor, select an adecuate torso
- Select SocketOffset (Not ADD)
- File->Export FBX animation (Export SkeletalMesh with animation)


2.- Import to 3DMax
---------------------------
- File Import -> Select FBX
- Import options, Advanced Options-> Units. Convert scale to cm


3.- Modify Asset
----------------------
Move The appropiated Bone

Bones:
********
PsiAmpHolster
SwordSheath
GrenadeLauncherSling


4.- Export from 3DMax
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- Select (Double Click) Every Bone in Root
- Select Control+Click Base Node (Now Skeletal Mesh is not selected)
- Export Selected
- Vertical Axis = Z


5.- Import to XCOM2 Editor
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- Create a copy of the current anim set, and open it
- File->Import FBX Animation
- Should replace the animation with the same name, some warnings can pop up, and should be ignored




I am kind of surprised that no one has commented the solution to this issue before.

I hope this information gets out to the modders and we can fix all the cool armors of the community