Image layering and blending with background

[Ofnuts]

You just need to copy/paste enough of the muzzle to cover the bone where it must be covered (but copy pasting more than this is fine... (green lines)), and then erase the bits where bone is visible, and here too you can do a rough job in most places, the only place where the cut need to be accurate is where the bone is in contact with the muzzle (red lines)).