There are two measurements in the bore of a barrel. The groove diameter and the bore diameter. The groove diameter is the distance from one side of the bore to the other at it's widest point. The bore diameter is the distance from one side of the bore to the other at its smallest point (tops of the rifleing).
The .270 is named what it is because .277 is the groove diameter (widest diameter)of the barrel and .270 is the bore diameter (smallest diameter)of the barrel.
So take .277 inches, which is the diameterof the bullet,and subtract .0035" per side of the bore to accountfor the rifling and you get .270 inches.
7mm is called 7mm because of the same reason. The 7mm bullet is .284" whichis also thegroove diameter of the barrel. Now subtract .004" for each side of the bore to account fir the rifling and you get_______________ .276 inches or 7mm. [

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This applies to most, but not all,of your center fire rifle cartridges.