So this thread's a bit (haha no pun intended) old hehe but I was trying to find a spanner bit for the XM's too. I have 3.5" XM-18's (4th gen), and I think the above posts were talking about the 3", because the #10 size is a bit too small.
For Wiha, they have bits here
http://www.wihatools.com/700seri/719span.htm but they stop at size #10. For the 3.5" like mine, you'll need a size #12 or #14 from somewhere else, I got the Irwin brand which seems to be at most places. #12 barely fits width-wise and fits fine thickness-wise. #14 is wider but is too thick so you'll need to file it down a tad with some sandpaper or a file, then it fits fine and is the preferred size. Up to you if you want to file it down or just go with the #12.
Wiha also has these
http://www.wihatools.com/300seri/305serie.htm but they're long and aren't as easy to work with when you're trying to turn both sides of the XM at once. If you go this route, get size M4, it's basically in between the #12 and #14 and fits without needing to be filed. I haven't tried the M5 but the measurements make it sound like it'll be too big to fit both in thickness and width.
Hope that helps anyone else who gets to this page. There's always the Hinderer Armorer's tool
http://www.rickhindererknives.com/rick-hinderer-store/gear/knife-tools.html ($75+) or the PSD tool [Nonsupporting dealer link removed] ($) if you're willing to part with the cash. I assume there must be some other type of spanner tool I'm not aware of? But the bits work for me so I'll stick with that. Some people said they just bend a fork to fit it? Heh, whatever works. Oh and sorry if I'm violating a forum rule with all the links, I'm not trying to recommend sales from a specific store, just giving advice.
