The 7mm designation is a reference to the bore size, or diameter. In decimal size it means this rifle has a barrel groove diameter of .284 inch.
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270
it is the measure of the width of the bore, given in millimeters or fractions of an inch. Thus you can have a .303 Enfield, or an 8mm Mauser.
Nothing. Gauge refers to shotgun bore
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have a gunsmith do this, push a slug of close dia through the rifle, and micrometer the bullet. this will give the true bore size.