I wonder how common it was to be mortally injured by a breaking spear? If the spear is too heavy it's slow to move and less likely to result in a quick kill, if

I wonder how common it was to be mortally injured by a breaking spear? If the spear is too heavy it's slow to move and less likely to result in a quick kill, if it's too light it'll break off without penetrating deep enough for a kill. Engineering trade-offs our ancestors must have pondered.