I just shipped my third 6.5 barrel away for a muzzle brake
. Enclosed are two photos of 6.5 WSM rifles. The first is a Model 70 in an MPI lightweight stock. It weighs 7.5 lbs as you see it with Sightron 2.5-10X 32mm in Conetrol rings and bases. I would not want it any lighter. It has a 26 inch Rock Creek 5R barrel in their #3 which is lighter than my Bartlein and Krieger #3 contours. All are fluted by the maker.
The Winchester magazine length is 3.15 inches. That works well with the 130 Nosler AB and will seat Berger 140 VLD Hunting and Hybrid Targets but deeper than I seat them in the Highwall. The "Short Action" Remington magazine is a problem. You could expand the magazine with the help of Wyatt boxes prehaps but I have not done this. I like Model 70s.
Before I chose 6.5 WSM I was a .264 Winchester Magnum handloader, going back to the late 70s. The published Winchester catalog data showed a 140 gr Spitzer at 3200 fps. Winchester used a bullet with nose before the channellure of bore diameter to ride on the lands and the base of the bullet was bore diameter to engage the rifling in the barrel. In practice it slammed into the rifling at an angle and had a lot of yaw to the target and 2.5 inch groups were common.
Handloading with Sierra 140 Gamekings or Nosler Partitions, 3150 fps is as fast as I ever got without the primer pockets bulging.
I tell you this because the capacity of the 6.5 WSM (Norma .270 WSM brass) is about 5 grains of powder less than the .264 Winchester Magnum.
Retumbo and VV N-570 can get you to 3200 fps from a 28 inch barrel with 140 gr Bullets in the 6.5 WSM. I can do this in my Highwall. I would doubt that you would see 3200 fps and without destroying the primer pockets in a 26 inch barrel.
If you send me an email I will send you a spreadsheet of my load and chronograph data.
A lot of the amazing chronograph readings that I see are from short radius chronographs that are not set up with the senors parallel to the bore.