All weapon packs are made by different artists and therefore they naturally have different looking hands and attire.
You can however change the texture, albeit with a less than ideal result as the advanced weapons pack has significantly higher detail than the old slingshot.
go to this folder: Game Guru\Files\gamecore\guns\medieval
to find the sling shot weapon. Hand_D is the texture, Hud_D is the model.
If you want to properly implement the slingshot with shaders so it renders correctly, there is a slightly more advanced tutorial you can use here:
LINK
