As someone earlier in this thread alluded to for a particular bullet, find out the minimum recommended impact velocity for the bullet you are considering to determine the maximum effective range with an external ballistics software package that will give you down-range velocities for a given data set (ballistic coefficient, muzzle velocity, altitude, etc.). Some cup-and-core bullets need at least 1900-2000 feet per second velocity at impact to expand reliably, while other bullets need more impact velocity and some require less velocity. Some people think it is a good idea to add a little bit of cushion (safety factor of say 100 fps) to what the bullet manufacturer says is the minimum impact velocity, and I wouldn't disagree with that practice.
If you want to push the envelope, you'll want to measure your muzzle velocity with a chronograph and verify your bullet's ballistic coefficient by shooting at various distances and comparing the drop at the various distances to what is predicted by the external ballistics software for the input parameters (including an accurate measurement of the distance between the center of your bore and the center of your scope).
There are a number of bullets for the .270 that will expand out to a distance of 400-500 yards with typical .270 muzzle velocities, but I suspect the number of bullets from a .270 that will expand reliably past 600 yards is probably pretty small. The Berger VLD is one I would be looking at, as others have mentioned.
