I've got an XR-100 in 204. Seems OK, though I have yet to do any real load work on it since it's been back from repair.
I'm somewhat disappointed with this particular gun as it seems to have a slow barrel - loads always clock in 200-300fps behind what the load book SAYS they should, although I can also increase my powder charges beyond what the load book indicates without any signs of pressure. I do moly my bullets, so this may contribute to the slower speeds and lower apparent pressures, though I've never before seen moly make quite this much difference.
Out of the box, the gun had a deep chamfer cut around the firing pin hole which manifested as severe primer cratering and leaking. Turns out this was a defect from the factory, and I had the bolt body replaced under warantee.
Hanging out on the Saubier sub-caliber forums, I learned that these sub-caliber cartridges have a reputation for being hard on primers, even under normal pressures. The suspicion is that they produce sharp pressure spikes that drop almost instantly, so that the MAP (maximum AVERAGE pressure) is still in spec. Therefore, you can shoot factory ammo or handloads known to be mild, and still have issues with the primers, despite the fact that everything is well within spec. It's been recommended that I try using Remington 7 1/2 BR primers as they have thicker cups than the Federal match I've been using. (CCI BR's are also thicker). I have yet to try and loads with the new primers. Slow powders seem to play a role here as well, it's better to go with powders on the slightly faster end of the spectrum.
This gun really doesn't care for factory ammo which is just as well as I don't shoot factory (except in the beginning before you could get .204 brass). So far, I'm working with Sierra 39gr BK's over Varget, though Varget seems a little slow for this round - I suspect something a little faster such as 4895 would be a little more suitable.
Before I had the defective bolt body replaced, I was starting to have some good luck with 29.1grs of Varget behind 39gr BK's seated out to the lands. This load is WAY over the max published loads, but velocities are in the right 3800fps range. This load produced some spectacular groups.
Anyway, I hope to do some more load work with it this summer and see what I can find.