I politely disagree... some calibers absolutely are inherently more efficient, the .277 isnt... its not about whether with enough time/money you can come up with one bullet that has a higher BC or not. A few manufacturers have already done this and the bullets are super long/heavy and require non-standard twist rates/throating and other considerations to run properly.The 6.5 is not inherently superior to the. 277. That is not an accurate statement. It actually has more potential. It's simply lacking development. Ballisticly the larger the caliber the more potential bc you have. (in bullet weights that can reasonably be produced and fired)
Google "Whats Wrong with 30 Caliber" by Bryan Litz, a summary of this article is posted on LRH somewhere already.
You can see this visually if you plot out a constant form factor (and by extension BC) on a scale of weight/caliber. The line intersects with commonly available heavy for caliber bullet weights for 6.5mm and 7mm projectiles (140/142gr and 175/180gr respectively) but to get a similar form factor/BC for .277 the chart intersects at 160grs which is not at all that common (at least until recently) and requires a faster than normal twist rate to properly stabilize. It's also a lot longer so we're talking custom throat, potential magazine/feeding issues, etc.
Again, this is why, even with commonly available 140 and 150gr .277cal bullets you have to run them much faster to get the same/better performance downrange. Why do you think that none of the .277 fans on here have actually compared the 270Win to a 6.5mm cartridge of similar case capacity like the PRC or 6.5-06 etc?
So yes, it can be done, but the 6.5mm does it more efficiently and with less recoil/powder burned, and if you need the "bigger is better" then you are better off stepping up to the 7mm.