As first noticed with the 275 Rigby, the designation refers to the barrel lands (.275") and not the grooves, a British way of doing things. In further review, the 270 Win appears to be the true 7mm as the bullet is 7mm (.2755") while the "7mm", .284" is actually 7.21mm. Can you shed any light on this?
Most US cartridge designations are dreamed up to sound sexy, not to convey definitive information of benefit only to a very small subset of users.
The most rational cartridge-designation system is the European metric system, which includes the caliber (bullet diameter), case length, and whether it's rimmed or rimless. For example, there's the 8x57JS, where the S means .323" bullet diameter (if there's no J then the bullet diameter is .318"). An "R" added, as in 8x57JRS means the case is rimmed, rather than "rimless." But there are still some oddities...and Americans can spend a lot of time sorting out the various 8x57 rounds.
Rifle/cartridge manufacturers from elsewhere tend to do whatever they want. One example is the .280 Remington, where .280" is neither the bore or groove diameter. Instead it apparently "sounded good" to the Remington PR people--who might have just averaged the .276" bore and .284 groove diameters.
The the first "version" of what eventually became the .270 Winchester appeared just before WWI, in a controlled-feed rifle Winchester's T.C. John designed in 1912 for sale in Europe. It featured a .287" diameter bullets (very close to the British diameter for ".28-caliber" bullets of .288"). But the war prevented it from doing anything notable in the market.
After WWI Winchester tweaked the design of both the rifle and cartridge. By then they'd also had a war with Mexico, which resulted in a bunch of 7x57 Mexican Mausers of varying quality entering the U.S. So Winchester reduced the diameter of the bullet, to prevent questionable Mexican Mausers from being rechambered for the new round, which appeared in 1925 in the Winchester Model 54.
Some of this info comes from the excellent book W.R.A. Co. Volume I, which is subtitled "Winchester Cartridges Used In Winchester Rifles, Headstamped Cartridges and Their Variations," written by Daniel L Shuey, and published in 2005.
But you can also find plenty of information in Cartridges of the World, which is now in its 17th edition. (The first edition appeared shortly after WWII.)
Yeah, a 38 Special sounds more impressive than a 35.
Just don't look too closely at the Weatherby Magnums.
Just don't look too closely at the Weatherby Magnums.
Or the plethora of 20th century.224’s that are named everything from .218 to .225 with a few redundancies thrown in for good measure, not to mention all the ones called .22 (something or other)