10mm
I have the parkerized version of this gun (and yes I have the Gold Match in stainless also). I have only had the Ultra about a week but I shot it last Friday, last Saturday, and at a match on Sunday (in the snubby competition).
The biggest surprise to me is the zero recoil. At least zero in the sense that it is indistinguishable to me from the recoil on the full size gun. I got rid of the Walther PPK/S in part because of the caliber and in part because the little sucker kicked like hell. The Ultra is harmless.
It was, at least in my hands, almost as accurate as the Gold Match (which may be more a comment on me than either gun). Others who shot it Sunday were also impressed with the accuracy.
Part of the reason I got the carry gun parkerized was so I wouldn't bust a gut when the first "ding" appears. There is a little bit of wear appearing after a week of slightly unorthodox carry methods (see "How To" post re same on board).
The disassembly and reassembly were a little bit awkward. First, the book that came in the box was for the Kimber Compact, not the Ultra, and the box included a take down tool. The book says put the take down tool in the little hole on the recoil rod. But, hey, they aint no hole there. If I hadn't already owned the Gold Match I probably would still be trying to get this bugger disassembled. But it worked. I used the take down tool to help push the slide catch out from the starboard side.
Reassembly was also a little tight. The recoil rod honest to God just didn't want to go back in. Force and violence to the rescue. Maybe things will loosen up some with use.
But, hey, after a week, I don't even remember what the Walther looked like. The Ultra Carry is a keeper. I hope it stands up over the next several months, but right now I am totally satisfied.