Does it have a "musket" nipple or the smaller #10 or #11 size?. 6mm likely if its #10 or #11. Take the nipple into a gun shop and compare them. My Zoave action is a "musket" nipple, and not sure what thread, but likely metric - I can measure if you need.
If you have some misc small metric and SAE bolts around the house, back out the nipple and see what threads in...