Requirements:

• https://github.com/BadDogSkyrim/PyNifly/releases
  
  

So, you want to import a weapon mesh and have it match 1:1 between the Weapon bone and how actor's hold the weapon in-game. We can do that, so long as you have the .nif mesh file located and PyNifly <= v19.4 installed (because v19.5+ breaks for Blender 4.2).

1. File > Import > Nif file with pyNifly (.nif)



2. Right click anywhere in object mode > Snap > Cursor to World Origin (just set the 3D cursor to 0, 0, 0 coordinates)



3. Copy these import settings. Optionally save them as a preset for "Weapon Import" using the top right + button.



4. Select the imported mesh > Right click > Set Origin > Origin to 3D Cursor



5. Shift click + select the mesh and shape keys, move them under Weapons (or where ever you want, really)



6. Clean up the extra unneeded objects



7. In Object Mode, select the new mesh (DwarvenSword:0 in my example) and rotate X to -90°.
8. In the object constraints, Add Object Constraint > Copy Transforms
9. Copy the following constraint settings and/or change the bone depending on if it's a magic node you want to target, etc.



10. Done. Your mesh is accurately imported and should reflect in-game.