6.5 mm bulletsare killing machines. I would consider it very hard. I just built a 6.5-284 and love it. Its a wildcat but you can buy all the components for it easily. Even the brass is headstamped correctly. The accuracy is legendary. The 6.5-284 is the cartridge of choice for long range benchresters.I'm pushing a 140 gr. bullet at approx. 2850fps out of a 26 in tube. With the ballistic coefficient of these bullets that makes for a very good long range weapon. Also the sectional density of the 6.5mm bullet is extraordinary making it a hard hitting & deep penetrating. If you build the 6.5-284 you will want to build it on a long action eventhough its a short action cartridge. To take advantage of the cartridge you need to shoot the 140's and let them out to the lands orvery close to the lands. If you do this with a short action you won't be able to load the magazine. So the solution is to use a long action.

If you are not interested in that, consider the 6.5-06, 260 rem, or 264 win mag. Otherwise, from the list you included I would pick the 280 rem, but would also consider the 280 ackley