What you're talking about has a proper name, a "Fowling Blunt," so called because it is, well, blunt and is used to hunt small birds, which otherwise would be too damaged to be used for food if a regular arrow were used. Oh, and it doesn't have to be heavy to knock someone out. Arrows pack a lot of punch. Though, as they were usually made from a ball of iron or lead, they could be quite heavy for an arrow.