Glamdring, although you know of my love affair (obsession?) with sectional density, there are a few drawbacks about taking the 6.5x55 on an elk hunt.
Shots at over 250 yards (even out to 450) are not at all uncommon when you are out for the mighty wapiti. Notwithstanding the truly excellent penetration of which the 6.5x55 is capable, this cartridge just lacks the initial velocity to carry the necessary punch at such distances. The result of being thusly undergunned is a frustrated (yet espensive) hunt and perhaps even lost game.
When you are out for elk, unless you know you are going to be hunting in very thick stuff, you should bring something that will pack enough punch out to roughly 300 yards.
You can either achieve this by equipping yourself with a flat-shooting classic such as the 7mm Rem or 300 Win (or any of the STW or Ultramags, of course); or you can do like I do and take a big thumper (338 Win or, in my case, 375H&H) and still whack them hard in spite of the more curved trajectory.
The question is not whether a certain caliber is excellent per se. The 6.5x55 certainly is, and it has plenty of historical aura, deserved accuracy reputation, good design, amazing SD, low fuss and low recoil. But the price you pay is in distance - and distance you will need if you want to hunt elk successfully.
Karamojo Bell, my idle, was considered a "long range marksman" in the heyday of the 7x75 and the 6.5x54MS. Yet, he would not pot his elephant at any greater distance than 100-150 yards.
Nowadays, such a shot at elk is considered short-range. The disappointment of losing a perfect trophy-class rack because you used a caliber that was only marginal is something I have experienced, and that I really do not wish upon any of my friends. And it more than offsets the sense of pride you have in a certain caliber as your "pet".
Although I have shot pretty much every caliber on the market - from the old 577-450 Martini Henry to the ultra-modern Lazzeronis, from the tiny .223 to the mighty Nitro Expresses, from the short 6mmPPC to the long 338 Lapua - I have my pets and I stick to them. The 6.5x55 is one.
But fortunately, so is the .375H&H.
A pet is a pet - no buts about it. If you own a lively, smart and fieldworthy Fox Terrier, let him make you happy with the game he will be most able to take. This way, he'll never let you down. Bf you want to run Brown Bears, get yourself a nice Irish Wolfhound. You can still love the terrier......