Let's say 6.5 creedmoor in Rem700 LA!
There gets to be a point where a chamber/barrel can only efficiently burn a finite amount of powder.
This is easily seen, with the introduction of new powder coatings that move the pressure curve further out into the barrel, usually increasing velocity and dampening harmonics, without increasing ignition pressures beyond safe limits.
The 6.5 Creedmore is a cartridge designed around these new powders.
IMHO, it will do anything the 260 Remington will do with bullets up to 140 grains, if the magazine will allow the bullets to be seated out far enough to increase the powder, depending on which one, by appx 5+%
You can't do that in a short action rifle.
I built a 6.5 Creedmore on a Winchester Mod 70 FWT, push feed, with a 1-8 twist standard weight barrel, 23 inches long for my sister to hunt Deer with.
This rifle started life as a 30-06 and didn't have a magazine spacer. I left it as is.
I built a 260 Remington with a similar barrel, other than it's appx a half inch longer, on a Remington 700LA.
They are both better shooters than the people shooting them.
Velocities are within 50 fps of each other, but that may just be the nature of the barrels.
No matter what loads I try, the results are the same.
Normally, the 6.5 CM will show appx 100-150fps LESS velocity than most 260 Rems.
By being able to load the bullets further out, the 6.5CM basicly duplicates the 260
The Win Mod 70 push feed has no problem with feeding any bullet weights or length.
Some people like to point out that there is a longer bolt throw. We're not in combat and even under hunting conditions, 1/10 second isn't going to mean anything.
The extra 4-5 ounces of weight isn't going to bother anyone and that weight actually helps to balance the rifle between the hands.
As for length. That's up to the individual shooters requirements, including velocity and bullet weights.