i shoot the 6.5x55 and have dropped five deer at ranges from 500-900 yards. five deer in as many shots at deer. and have passed many shots up due to bad weather or no clear shot.
deer are medium game and the 6.5 proj is sufficient for a clean kill, if you are hunting elk or moose, that is different. obviously a 300 yard shot and an 800 yard shot require not just the right load, gun, and optics but most importantly skill to take the shot at those ranges from a cold barrel. if wind is an issue, wait. better still if u get that chance go out and hit gongs on a windy day and you will gain more skill. the 6.5 round bucks the wind with the best of the 30 cals, if not better, so to say it was caliber choice that fouled the kill is wrong.
i have shot goats many a time at 700-800 yards across a steep gorge in winds up to and including 25MPH. but i also practiced at those cross winds and build up data, not just rely on software for my windage. when the wind picked up harder, i chose not to shoot, simple..
practice, practice, practice....more important that caliber choice...
any of the 6.5 rounds is great.....if a bigger caliber gives you more confidence, sure go for it..
one bit of equipment i can't live without anymore is the cosine gauge..it will explain many near hits.. and clear misses...