After posting my model and re evaluating the image you posted, i realized there is an error in concept that can make it harder to do with this method. The transition from the butt to the grip ismore or less tangent in the image. Mine is “smooth” but there is a definite shift in size.

I am currently out but will re look at a different way for you to achieve a good NURBS result to move forward

The rest of what i said still stands