I might be OCD. I have been contemplating the heavy shaft dilemma. I only shoot wood and find it hard to make heavy shafts that are not too stiff. Seems you would want a filler material that flexed with the shaft, but did not absorb any energy on impact. After thinking about what might work for a week now, I think I know what I would try, D3o. It's an infused polyurethane material that is somewhat soft and flexible, but stiffens on impact. They use it for impact protection, cell phone cases, etc. I demoed some MTB knee pads that had the stuff for padding. It was flexible while riding, but if you smacked it with a stick or rock, it would get hard on impact.