The new Ritter & Stark SX-1 Modular Tactical Rifle was designed primarily for military applications, but it may be available to civilians in the US soon. It is not only capable of switching calibers (308 WIN, 300 WIN MAG, 338 LM), but it can also switch across several magazine standards as well.
One of the most unusual features is that the barrel is rifled in an EDM to avoid transfer of thermal and mechanical effects to the material. They are thus able to guarantee 0.5 MOA for 2-3 times the round count of competing systems.
For more info, visit: http://www.ritterstark.com/rifle.html