The difference in naming is whether you are measuring the Bore diameter(from land to land) or Groove diameter (groove to groove). So for a standard 30 Cal the Bore is .300 and the Groove is .308. for 7mm its .275ish and .284ish. A .318 is the bore and .330 is what I think the Groove Diameter is for a .318 WR. Bullets match the groove diameter so the lands(rifling) grab on a force the bullet to rotate.

At least that's how I understand it. I'm sure smarter people will correct or add to it. It does seem the Brits are a little more inconsistent with naming. Especially with the brits liking to produce novel diameters for their branding. .375 is another named by groove diameter while Holland released the .275 H/H at the same time based off bore diameter. I guess to match the .375. Overall mostly by bore and that matches how they used to say "rigby's .275 bore" or such on barrels and ammo boxes. The differences in naming in America are mostly due to the multitude of same caliber cartridges we redundantly invent.

.333 Jeffery shots a 300gr bullet about the same speed a 318 shoots a 250 and then the .333 shoots a 250gr 2500+. Flanged is 50-100 fps slower than rimless.

And I want a .333 so bad. bolt and double. My dream medium bore