I managed to make a prone feature but it looks really bad because if you want to shoot it forces your torso up in the air and you look really weird, like your legs are laid flat, but when it gets to your stomach you go vertical as if standing. If you can get rid of that while still being able to fire it would be good.
Anyway you don't need a prone feature for a musket mod, you couldn't easily reload like that with a muzzle loader.