The .270 has a .277"-.278" groove diameter. 7mm's are .284" (in the U.S.) but can be anywher from .284" to .288" in European cartridges. There are LOTS of 7mms, so I assume by 7mm you mean a 7mm Remington Magnum?? Shooting a 150-grain Nosler Partition or other tough, deep-penetrating .277" bullet at a MV of 2800 FPS or more, the .270 Win. is plenty gun for anything in North America except the great bears, and quite a few of these have been killed with the .270 as well. I love the .270, used it for years, and would not be too put out if I still had to hunt with one.

These days, I use a 7mm Remington Magnum. Mine has a 26" barrel, and shoots the 175-grain Nosler Partition bullet at a MV of 3050 FPS using 70 grains of IMR 7828 in Winchester cases. Obviously that is a little more powerful than a .270, and it does a very good job! Yet either one fails if you put the bullet in a bad spot! There is NO CARTRIDGE in this world that can be fired from the shoulder by a human that will compensate for poor shooting!