It is a bit curious that ruger chose the .311 bore as opposed to .310 which is what I typically see in other US manufactured barrels--my guess (and I'm sure someone will quickly correct me if I'm wrong) is that they figured the accuracy impact on .310 cartridges would be minimal vs. being able to operate comblock bulk mil-spec cheapo plinking ammo.
Hornady's dies come with an expander ball for both .310 and .308 and they say you probably won't need any adjustments other than switching them out to seat a .308 bullet. I personally don't see the advantage in doing this--I know there are gobs of .308 bullets with much higher BC pointy boat-tail/spitzer designs out there but much like the 300 blk when you stuff a longish bullet into the case you're going to be encroaching on powder capacity and performance. The better performing powders I've used so far for the x 39 tend to be stick-like powders--though the new CFE BLK, being a smallish spherical powder that works well in the x 39, has been a game-changer for the reloads I've done with it. I still haven't tried 1680 but will do so shortly. Stick type extrusions have their own issues with getting them into the case and bullets seated in compressed loads. The AR 47 I recently put together using a cheapo PSA upper is IMO a genuine 100 yd MOA shooter with the right cartridge. Cavity back makes a .310 125 gr monolithic bullet which features their "cavity pocket" at the base enabling a grain or so of extra powder capacity to be used and reportedly has a very wide petal opening velocity range--starting at something around 1100 fps.
Ruger is generally very savvy about their new rifles--again, just a guess, but I'm sure they wouldn't bother bringing the x39 out on the predator ranch design if it was not an inherently accurate rifle--at least superior to their semi-auto offerings.