At extended ranges, what comes out the end of the barrel, and what the barrel is attached to ain't near as important as experience and a capable shooter who can read wind, light, and mirage, and adjust accordingly.
Whatever you choose, the rifle does need to be a consistent performer.
Having shot 1000 yards successfully, I couldn't agree with the above statement more. Pulling the trigger is only 1/2 the game, the other 1/2 is reading the conditions. This is assuming that the rifle being used is capable and consistent (a "shooter").
As far as a Handi caliber for shooting 1000 yards, I would choose either of the offerings in 7mm and probably lean harder toward the one with a longer barrel. That said, between all of the rifles mentioned (25-06, 270, 7mm-08, 280, 308, 30-06) there wont be enough of a difference on the ballistics tables to really say one will stand out over the other. In a capable shooters hands they are all good calibers capable of cutting paper at 1000 yards. The major difference in the cartridges that I just mentioned I think would boil down to recoil, and the 25-06 and the 7mm-08 would be favorable in terms of recoil in my opinion.
And yes, as mentioned in some other posts you also have to consider the stated better BC's in the 7mm bullets and the availability of bullets as well.
In closing, take a look again at the quote I included from Walton. Although it is fun to talk about which cartridge is better than the other, it really doesn't make as much of a difference as we would like to think. It's more about rifle consistency and shooter capability than how big of boom it makes or how flat a path it takes to the target if it gets there consistently.
Anyone remember / see the famous shot made by Fred Bear when he took a Brown bear taking a shot arching his arrow over a big rock? If that weapon shot flatter, he could have never made that particular shot. But, given that shot multiple times I bet he could CONSISTENTLY have hit his mark as he was a capable shooter (to say the least) with a consistent weapon.
LM