Thanks very much Tim, the more I look at this the more I think of Congo as well, and especially the talismanic possibilities.
Kate,
I'm with you, I wish there was a simple book to concisely show us each tribe and exactly which weapons were used by each. However in ethnographic weapons study nothing is ever simple. By thier very nature, tribal people move, diffuse and are constantly in varying degrees of change and movement, still maintaining key traditions tempered by encroaching or encountered influences. In Africa, there is a considerable degree of artistic license and symbolic application to weapons, thus the wide range of variation in the forms of weapons used, and as you have noted, depending on the intended use.
Spears come in many sizes and are used in numerous ways, so many shafted weapons of larger size mistaken for spears are often actually employed as arrows. This is more typical of Oceanic weaponry, especially that of Melanesia
(see "Arrows of Melanesia: A Neglected Art Form" by David Skinner, in "Tribal Arts" , Summer 2000, Vol.VI, #2, pp.86-99). The heading of the article notes, "...they're not spears".
As we always say here, 'more research is needed', so we'll keep looking
All the best,
Jim