6.5 Rem Mag
284 Win
Here's a couple of drawings of each base cartridge. It seems the 284 is a little narrower at the base and shoulder, with a little more taper and a longer neck, but has a longer base to shoulder length due to its sharper shoulder angle. So the capacities should be real close with the edge probably slightly to the 6.5 RM, although probably not enough to make a big difference.