Re: x-Bolt 15 or 20 moa rail
I thought I had an option I've come across, but everything is listed as short action. The only full length picatinny bases I've seen for the x-bolt in general are from Murphy Precision or EGW... both of which only list for the short action. I think I've seen before that Murphy will custom make them for you, might be worth a call.
The other option I was looking into was getting 2 piece bases and for the extra moa use the Burris Signature rings with their offset rings. I've seen good comments here about them, but no experience myself.
This is the biggest problem with the x-bolt... not much out there for them.
Sorry I didn't have THE answer, but hope this helps at all. Good luck and enjoy your new x-bolt.