From Dave Emary, Chief Ballistician-Hornady:
The 260 has a lot of drawbacks. It was designed from the get-go as a hunting cartridge, maximize case capacity, not meant for heavy bullets. It has a lot of body taper and a rather shallow shoulder. Neither one of these things are ideal for a cartridge “centering up” well in the chamber and minimizing Principle Axis Tilt” (PAT) between the projectile and the bore, improving accuracy potential. The 6.5 Creedmoor has minimum body taper and a 30 degree shoulder. The 260 has a shorter throat and a 3 degree lead where the Creedmoor has a longer throat and a 1.5 degree lead. The throat in the 260 forces you to seat longer/heavy bullets very deep in the case negating any case capacity advantage it has and the bullet does engage the rifling as straight with no throat and a 3 degree lead. The Creedmoor has a .2645” lead which again limits the possibility of any PAT, further increasing the accuracy potential of the cartridge. The biggest drawback of the 260 is it was designed with a 1-9” twist pretty much eliminating the possibility of using any bullet more aggressive than a 140 gr flat base.
If we were to have fixed all the problems with the 260 to make it an acceptable “Match” cartridge it would no longer have been the 260 and wouldn’t have been a factory cartridge anymore. Thus, start over and do it right from the start.