6mm Hagar is a longer case than SPC, works really well with lighter bullets.
6.5x43 is limited in ogive length.
Case capacity is not more.
I'd almost be interested in a 6.35 version (quarterbore), except it doesn't do anything 6.5 Grendel can't. I have an affinity for the .257 bore since my grandpa's favorite cartridge was the .257 Roberts.
Like .277" though, it is only .007" away from more popular bore diameters that have much better projectile selection.
In the case of the .257", .264" just offers so much more in projo type and numbers, with much higher BC's and sectional density.
With .277", the .284" (7mm) is the same, way higher BC's and projo selection, in the same weights and way higher.
I personally like the option of using 129gr and 130gr pills in the 6.5 Grendel, and actually load those fairly regularly in brass that is ready to go with the 129gr SST and 129gr ABLR.
Here is what the 6.5 DTI looks like with a 100gr Nosler Partition:
NRA Basic, Pistol, Rifle, Shotgun, RSO
CCW, CQM, DM, Long Range Rifle Instructor
6.5 Grendel Reloading Handbooks & chamber brushes can be found here:
www.AR15buildbox.com