ATH,
Your asking questions that deal more with cartridges then bore diameter. The 6.5mm is a great calibe rfor long range shooting but it will not run with the large 300 magnums ballistically when 220 to 250 gr bullets are shot.
One exception may be a new bullet in the works from Wildcat Bullets which will be a ULD design in the 160 to 170 gr class. Still figuring this one out. This bullet will be disigned for use with my 6.5mm Allen Mag.
While the big 300s will ballistically top the 6.5mm rounds, they are much more difficult to shoot because of increased recoil, and muzzle blast.
This is what makes the smaller 6.5mm rounds shine, they are very good ballistically, they are accurate, and they are easy to shoot, especially volume shooting.
They generally burn less powder then the 300 mags so barrel heat is less of a concern. I would not say they are any easier on the throat. The 300 Win Mag is not overly hard on a throat and some of the medium to larger 6.5mm round are far harder on the throat.
Personally I would say the 6.5mm rounds are plenty powerful for taking deer out to 800 yards. Farther in some cases. Larger game liek elk, I knwo it is done and effectively at that but I like a little more frontal area on elk, especially past 500 yards or so. A properly loaded 6.5mm round getting 3000 fps with a 140 gr class bullet or heavier will certainly harvest elk though.
Again this depends on the cartiridge used more then the caliber. Let me explain. You can take a 260 Rem chambered in a quality rifle and shoot the 140 gr class VLD and ULD bullets with extreme accuracy even at very long ranges. This round will last several thousand rounds before needing to be retubed.
6.5mm rounds in this class include the 6.5-284, 6.5x55 and others in this size.
On the other end of the spectrum, you have the large high performance 6.5mm rounds. They start with the 264 Win Mag and include rounds like the 6.5-300 Wby, 6.5 STW and my 6.5mm Allen Mag.
These rounds offer very high levels of performance but at a cost of higher recoil and shorter barrel life. My 6.5mm Allen Mag will burn 95 to 105 gr of powder to drive 100 to 160 gr bullets to 4150 to 3350 fps respectively.
Now barrel life is certainly abreviated compared to the smaller 6.5mm rounds but for a big game rifle, they re simply awsome and offer plenty of barrel life for a coupel of lifetimes for the average big game hunter.
Ballistically, with the correct bullets, these big 6.5mms will handlily out perform the 300 mag rounds.
Remember though that extreme performance, ballisticially or terminally mean very little if you can't put that bullet on the mark. This is why the 6.5mm rounds are so popular for long range shooting. They are easier to hit with then a heavy recoiling 300 magnum round.
Good Shooting!!
Kirby Allen (50)
