I'm with Bart B. this guy sounds like a gun store know it all. Strong opinions. but some wrong conclusions.
As far as lethality goes, it comes from Accuracy (the 6.5x55 general has it), Penetration (the 6.5x55 has a high sectional density for good penetration), and finally disruption (how big of a wound channel can the bullet expand).
If you look at a 140gr 6.5mm Sierra Game King launched at a sedate 2600 fps from a 6.5 Swede, compared to a 7.62 150gr Sierra Game King launched at 2800 fps from a 308 Win, the ballistic drops are within 1.3 inches at 300 meters, and energy remaining at 500 is actually higher for the 6.5 Swede.
Machine guns are awesome until you have to carry one.