I drilled out another head today using the 10mm drill bit by hand , as it was drilled to the depth I wanted and just gave it a pumping action before I withdrew the bit.
It seems to have worked okay the screw joint went in flush including the flange without cracking. I think I may have overtightened the 1st one I did pulling the flange in to far ,putting to much pressure onto the wood.
I have already ordered a 11m bit dearer than yours Gloops also from eBay think that extra room will prevent any chance of splitting the wood whilst inserting it .so if your using them Gloops try not to overtighten them .
this is the female mallard I have started on had a good couple of hours without being disturbed
I will cut a threaded bar to length and insert it straight into the shank and epoxy a bone collar onto to it at the same time.
The mallards head I placed a thin buffalo horn collar onto the head and crewed the screw joint through the collar straight into the topper ,had to countersink the topper slightly so it would sit flush with the bone collar ,seems to have done the trick okay. It looks good with contrasting collars
The hairline crack in yesterdays effort seems to be okay I unscrewed the joining screw put a touch of epoxy around the screw and refitted it not quite as tight this time so it should hold, The crack hasn't spread
I had considered using a screw joint in both the shank and topper but I don't think there would be any advantage in it.