The ancient 6.5x55, or the modern .260 Remington, or the brand new wunderkind 6.5 Creedmoor?
In fact, to this must be added the old 6.5x54 Mannlicher, the old 6.5x57 Mauser, the old 6.5x58 Portuguese, the old 6.5x64 Brenneke, the .264 Win Mag - not to mention the old Carcano and Arisaka. Had a high velocity bullet been the requirement all these must look up to the old German 6.5x68 Schuler. Fact is, no matter the amount of hype,"there is nothing new under the sun" - it had all been done in Europe more than 100 years ago; Let us view the "improvements" (if any) by the modern offerings:
As an outright statement regarding hunting, an honest review must immediately begin with: "Ignore the little differences in case geometry (including the dogmatic 'shoulder angles') of the different cartridges and take the rifle with the nicest wood and neatest appearance an most reliable action; that one which draws your eyes immediately and speaks to you - and particularly if it shoulders easily with the iron sights naturally lined up. No matter the cartridge it takes it will be as good - in fact exactly the same - as any other one in the list".
Forget about the infatuation US shooters have with engaging in endless, worthless, repeated and often downright silly arguments on the .260 Rem and the 6.5mm Creedmoor (or the .308W and 30-06 for that matter). At that immature level of argument the 6.5x55 is better than both the 260 and Creedmoor - and the 6.5x57 is better than the Swede, and the 6.5x58 Portuguese is better than 6.5x57, and so on. Silly splitting of hairs; an energy wasting infatuation with creating paralysis by analysis.
In practice they are all the same. Once the bullet is flying (in fact even as it leaves the case mouth) the geometry of the combustion chamber of that gas generator that had spitted it out has nothing to do with its performance on the animal you are shooting at. Then it is only him and his design ability to withstand the heat release into its mass during a sustained impulse on the low shoulder of the animal by the kinetic energy it had carried.
No matter the distance you shoot your mule deer at - the bullet from a 6.5x58 Portuguese impacts the same as would a 6.5x57 Mauser had the deer been two yards closer.
If the deer was another two yards closer a 6.5x55 bullet would impact as the 6.5x57 did that two yards further.
The 6.5x54 Mannlicher impacts the same as the Swede would have done two yards further.
The .260 Rem bullet impacts the same as would the Mannlicher have done one yard further.
The 6.5 Creedmoor bullet would strike three yards closer as hard as the .260 Rem would do that three yards further.
The US gun media, when referring to the performance of the old European 6.5mm, 7mm and 8mm cartridges seem to have colluded to only list their original performance with the old, too fast propellants of 100 years earlier. Furthermore, the informal and inconsistent specifications adviser SAAMI downgraded each one of these to such low pressure levels that they pale in significance compared to their new US counterparts.
CPI demands that every single rifle (yes, every single one) with its cartridge has to be proof fired at 125% of the maximum pressure applied for. Careful measurements are carried out to confirm all specifications with zero tolerance for any plastic deformation.
The 6.5x55 '94 Carl Gustaf is fired at 71,000 psi, as an example. This means that hand loaders consider 60,000-62,000 psi as a safe working pressure for these rifles and cartridges. Factory loads will be at least 10,000 psi higher than US factory ammunition for the same cartridges.
To meet the same quality standard in the USA every single rifle chambered for the .270W and a cartridge from each manufacturer should be fired at 81,350 psi before being sold.
One example is the 6.5x55 Swede. In the real world its case capacity allows it to outperform the .260 Rem by about 50 ft/sec with 140gr bullets and by 90 ft/sec with the long 156-160gr.
In the virtual world where any European action prior to the Mauser 1898 is deemed by SAAMI to be weak, the superbly strong Carl Gustav 1894/1896 is limited to 51,000 psi while the .260 Rem and Creedmoor are rated for 61,000 psi.
In the real world of Europe and Africa the ability of the 6.5x55 is well known and respected; its action is respected for what it is - one of the finest ever made with the strictest quality specifications, quality control processes and quality assurance testing.
With these base lines we can review the following table and evaluate the value of the modern offerings: