You cannot ream a 7x57 chamber to .280. Problem is, the chamber neck is smaller on the .280. There will be a residual neck left in the chamber from the original 7x57 chamber. Problem with shoulder could be another issue as was mentioned. If so, simply go to a .280 AI instead.
Actually you can, but your absolutely right on the neck.
I have a .280 reamed from a 7x57.
The smith turned one thread off in an attempt to not get that "residual" left. As it is there is approx 1/8" of that oversize neck upon fireing. It hasn't been an issue, I simply only neck size as far as the residual.
I still can't figure how people get any amount of accuracy from a cartridge(7x57) with that amount of slop in the neck dia....well at least I couldn't, that ream job tightened my group sizes in my rifle by 3" or so.