Well that settles it. I can put the 243 away and hunt Elk with a 6.5 Carcano as long as I use a specialty bullet.I disagree. Bullet construction is an important variable and it isn't magic. Some bullets are constructed more sturdily than others. The more sturdy the bullet the less it expands at a given velocity and the deeper it can go. The more weight a bullet retains the deeper it can go. Bullet construction is often tailored to certain velocities.
For example many 30 caliber bullets meant for use in 30-30 have thin jackets and are meant to open up at relatively lower velocities. Same caliber, same weight, and different application. Few if anybody would think of these as ideal for big game let alone dangerous game. They are however good for deer.
Hornady is one of the most forthright of the bullet manufactures stating what game it is meant for and appropriate muzzle velocities.
Hornady lists NO BULLET under 6.5 caliber rated for big game. The 6.5mm 140 grain SST and Interlock, the 143 grain ELD-X, and the 160 grain Interlock are all rated for big game. The 129 grain Interlock, which is a fantastic deer bullet, is not rated for big game.
Sierra is the same deal. The 6.5mm 140 grain Gameking is rated for "heavy game." No lighter bullet (like the 130 grain Gameking) or smaller caliber is rated for "heavy game."
Lapua also lists the 6.5 mm 155 grain Mega bullet for big game. Again no caliber smaller than 6.5 is rated for big game by Lapua.
Speer and Nosler are both more vague, though the 140 grain Hot-Cor is considered a medium game bullet (I've called Speer and asked).
Many would consider 6.5 under caliber for big game. I say it is a minimum based on the very makers' recommendations, who, after all want you to be satisfied with performance. I certainly wouldn't chance things buy using a bullet not even recommended by the manufacturer for the intended purpose, and to date I have found no maker positively rating the use of .243 caliber or even .257 for anything besides medium game.
To me this is a decisive disadvantage to .243 Win vs 6.5 Creedmore. Both are based on the same case, the .308 Win, though unlike the 260 Rem, the 6.5 Creed can actually handle the long heavy bullets and has an appropriate twist rate.