Any ballistic advantage the 6.5mm has over the .277 caliber is based solely on bullet availability. The Ballistic Coefficient (BC) of a bullet is it’s sectional density (SD) divided by its form factor. Compare a group of different caliber bullets that have the same shape and those with the highest sectional densities will have the highest BCs, i.e. the largest calibers. Using bullets of the same shape, any .277 caliber bullet would have a higher BC and a SD than its 6.5mm counterpart. It is the same with .257 caliber bullets compared to 6mm.

Consider the example of the Hornady 147 gr 6.5 ELD-match bullet given above. That bullet has an SD of 0.301. A 161.5 gr .277 gr bullet would achieve the same 0.301 SD and if made to the same shape as the 6.5 mm ELD bullet their BCs would be equal. On the other hand, it would be more comparable for Hornady to make something like a 168 gr .277" ELD and that would trump the BC of the 6.5.

But the market is what it is and bullet makers are not being called upon to make match bullets in diameters between 6mm and 6.5mm or between 6.5mm and 7mm.