Nice rifle! While I'm not a 40X expert, I'm going to take a SWAG that it's a 90's or 20 ought rifle from the SS Receiver and the KS stock?
During that era, I do know that some PD's were using 40X's as sniper rifles, so it makes sense that a NATO chamber would be used as those were expected to digest all kinds of ammo. I'll chalk up the NATO chamber in the XBR as one of those "things that make you go hmmm?"
Bernchrest accuracy is a little different and I have a tight neck reamer (334) specifically for that.
greydog's statement just adds to the "hmmm"
Random question; Your rifle is sans clip slot right?