if rechamber from 7x57 its the 280

you can't rechamber 7x57 to 7x64. look at the shoulder diameter, with 7x64 you get a step in the 7x57 chamber. the 280 is having a bigger shoulder diameter and so I assume it cut the 7x57 chamber completly away.

if the rifle works safely with 7x64 brass I would say its an oversized 7x64 chamber and use it this way.