Good question. Care to make an argument as to why? Im looking for something to fill the hollow grip on the stock, because it flexes. Concrete seems like it might be a heavy option, but also seems like it will be rock solid (I know, bad pun)
Does Fiberglass resin shrink as it cures?
Don't carbon fiber arrows have some flex to them? I didn't realize they were super rigid? I saw some in Princess Auto in the surplus section a few weeks ago and they seems pretty flexible to me - certainly more flexible than the aluminum ones right beside (technically I think they were crossbow bolts, not sure if that makes any difference to the flexibility, maybe they were just cheap??)