Re: 280 Rem.
I seem to be jumping all over this forum and only because I have owned or still own many of the calibers the new members are inquiring about. If I can help in any way, I will try to do so anytime.
The 7mm Bullets have a very good BC especially when going on up to the 168 Gr Sierra MK. The 162 Hornady Match is good if you can find the older ones. Your barrel twist should stabilize them.
I know of fellows who have shot in 1000 yd matches with the 280 and 280 IMP.
I still own two of the 280 caliber rifles along with the 7mm mag and the 7/300 Weatherby and can give a good comparison.
As far as killing deer size game, especially eastern white tail , the energy level is still enough to kill at 1000 yds, believe it or not, as I have seen it done. I would think that if you didn't have all the equipment such as Big eys, good Rangefinders and a good hunting partner to spot for you, don't try that range. The point is, the energy is there. A more relistic range would be in the 500 to 600 yds and with a know distance to the target
Good luck and good shooting
I guess that's all for now.