I'm not an AI fan for most cartridges unless there is an intrinsic improvement in accuracy as there is very little velocity gain and I hate fireforming cases. That said, I have had both(.260 rem and 6.5x284) and for 600 yds, there is no reason to go the more expensive 6.5x284. Also, I would recommend against building a 284 case rifle on a short action. If you use the longer bullets, they will have to be seated so deep the the ogive is INSIDE the case neck and this is not a good thing. I just tried it with a couple of my favorite bullets. The Sierra 142 MK makes a loaded round length of at least 3.00", the 130 Norma-2.88" and my favorite, the 108 Lapua-2.85; all of which are longer than a short action (700) magazine box.
Go with the 260- not sure your choice of stock pattern though-that stock is better suited for a much heavier barrel than you proposed.
Barrel life with a 260 is much improved over the 6.5x284 but barrel life is really a pipe dream for 99% of the shooters out there!
