Great input and thanks.
Yes I can see the logic of not using silver in a projectile that one would be unlikely to retrive.
Let me add the following observations with my attempt at explaining them.
(a) The raised ring A1-B1 has a flat surface towards the octagonal section which we assume is where the shaft was fixed. Hence the obvious reason would be to prevent the shaft sliding forward in the event of a hard impact.
(b) The octagonal part shows that anteriorly i.e. just after the raised ring it is narrow and gradually widens towards the posterior, becoming slightly narrow once again at the furtherst end. If this is intentional, once the sleeve is in place, it would prevent the head flying off leaving the shaft behind in the event of a forward motion applied to the shaft.
On the above observations I am of the opinion that it was projected in some way with some force though that does not automatically relate it to the cannon situation. Of course that does not explain the silver inlay unless there was some talismanic object as Jim has indicated.
I assume the previous discussion on gun arrrows at
http://www.vikingsword.com/vb/showthread.php?t=7158 and in particular the link
http://www.koreanarchery.org/classic/bhwangja.mpg would have been looked at where in the winged arrow, the head is sunken into the shaft. I cannot open the other links therein.
Best Regards.