The 6.5-06 works pretty good. I have one. Douglas will
make you a barrel in that caliber. They did my barrel. The
6.5-06 is very close to a 25/06 or a 270, but you can get
it with an 8 inch twist barrel. You can load up to 160 grain
bullet, way beyond the 25/06. You can also get quite high
muzzle velocity from a 140 or 160 grain bullet using something
like H870 or Accurate 8700 powder as well as some other
real slow burners, and you can get the velocity at pretty
low chamber pressures with those powders. You can get
3000 fps at low pressures with a 140 grain boattail, using
a slow powder like I mentioned and the balistic coefficient
is very high, I just loaded up some reduced loads with
4062, 42 grains, that gives 2710 fps, and that ought
to give a barrel life of about 10,000 rounds. Loading the
6.5 is very easy. Use 25/06 brass, and just seat a
boat tail bullet in it to slightly expand the neck to size.
Another 6.5 I like is a 6.5/257. Douglas sold me a barrel
in that also, and it comes within a whisper of being just
as fast as the 6.5-06, Likewise easy to load, just use
257 Roberts cases and seat a boat tail bullet in it the first
time you load it. It can also be used in a shorter action,
being the case is 2 1/4 inch long. You can actually use
6.5X55 loading data for it. they are so close in case
size, 1 grain of powder difference. so why go for the
6.5/257 instead of a 6.5 swede? The Roberts case is
same dimension except for the neck as 7X57 and I think
maybe a 6mm Remington also, gives more choices in the
brass to use. The 57 case length may work better in
some mausers, lots of them used 57mm cases, after all.