Arrows; light versus heavy

[D B Campbell]

Biblades and trilobate are often less thick (7-8 mm) then heavier bodkin types (9 - 11 mm). Could this indicate a difference in use ... the heavier arrows being used at closer range, packing more punch (mass), to penetrate defensive devises like mail and shields?

If we are talking about the same thing, Robert, the bodkin type are usually interpreted as catapult arrowheads, like this one from Vindolanda.