I'm a little skeptical about using a 270win @ 600 yards for shooting whitetails. Sounds more like a job for a 300Wby or equivelent.
However, under ideal conditions and with perfect shot placement I guess it can be done.
The Hornady SST or Nosler B-Tip should both be capable of good longrange accuracy and dependable expansion at low velocity.
You may also get a few recommendations for the Sierra 135gr matchking bullet. High BC and very accurate. [img]images/icons/wink.gif[/img]
I would not expect the SST to be demonstrably different in expansion than the B-Tip's. Several field tests that I have read indicated that they performed nearly identical. Expansion depended more on animal, angle, velocity @ impact and so on.
The perfect bullet might just be the Swift 130gr Scirocco. The .284" Scirocco's shoot great in my 7STW.
With a 270 135 Gr Sierra MK at 3000 FPS muzzle velocity, you will have 1779 FPS and 949 Foot pounds of energy left at 700 yards.
Plenty to kill a white tail deer and a bunch left over. It don't take much to kill a white tail deer. Bullet placement is key.
When Longrange hunting began in the Pine Creek area of PA after WW2, the only rifles the fellows used were the 270 and 30/06 and they were making kills regularly at 700 and 800 yards. They didn't have a bullet as good as the 135 gr MK either.
I have used the nosler BT and found them just a bit to explosive. I have shot 1 deer at 550 yards and many over the 400 yard range. When the conditions are good and I do my job the 270 seams to work well at these distences. But, I don't think I would push it any farther. Thinking about building a 30 cal LR rifle when I get the time and money.