There is a sticky in this forum entitled "Introduction to Practical Long-Range Rifle Shooting - Resources". There are several that discuss cartridge selection and comparison in depth. For now, let me direct you to Page 2 of this one:
| Practical Long-Range Rifle Shooting, Part I - Rifle & Equipment
The general criteria for selecting a cartridge for long-range shooting is that you want to minimize wind drift while keeping a reasonably flat trajectory. BC is the most important factor in wind drift. Once you have a cartridge, you should pick a load using the same criteria: pick the highest BC bullet you can shoot while still having a reasonable fast muzzle velocity.
The problem with .277" and .25" bore sizes is that the selection of high BC bullets is very, very poor. In the .30-06 case size, the best factory (non-Ackley Improved) cartridges for long-range shooting are: 6.5-06, .280 Remington (aka 7mm-06), and the .30-06 itself.