I just happened across this image of King Henry VIII as David confronting Goliath - this is from Henry's private psalter of 1540 in which he appears as David in several different scenes (praying, reading and playing the harp). Henry apparently compared his life to that of David in the margins of the psalter (Wooding, L., Henry VIII 2009, pp. 188-189). Fascinating to note that he appears to be holding a staff sling rather than a hand sling - see also the rock on Goliath's forehead. Could this be the moment that the rock hit, or perhaps it was embedded seconds before?

Hopefully the image works!