.280 Ross
The .280 Ross, also known as the .280 Nitro, .280 Rimless Nitro Express Ross and .280 Rimless cartridge,
is an approximately 7mm bullet diameter rifle round developed in Canada by F.W. Jones as a ... Wikipedia
Bullet diameter: .287 in (7.3 mm)
Rim diameter: .556 in (14.1 mm)
Case length: 2.59 in (66 mm)
Case capacity: 76 gr H2O (4.9 cm3)
Neck diameter: .317 in (8.1 mm)
Base diameter: .534 in (13.6 mm)
Shoulder diameter: .404 in (10.3 mm)

According to this, yes, the .300Win.Mag. case would work without having to touch the belt. The case body will form out when fire forming.
The only problem is rim dia. but if the extractor still works, fine and dandy.
I thought the choice of a .287" bullet, being a bit strange.