Misleading title... I was reading Osprey "Hasting 1066", and it describes the sling as being only useful at a range of thirty metres, but could kill.  However, the book, in the glossary section, describes a self-bow as a "bow considerably shorter than a longbow".  Not, in fact, the case.  So I believe the author needs to check his facts, weaponry wise.

Osprey has been known to be hit and miss.....

It interesting that he noted the slings lethality at ranges of sub thirty feet.  Could you forward me the original text?

Chris