Ruger wants to sell their rifles in European (GB included) countries too, where their barrels have to undergo official proof by a proofhouse. So their barrels have to conform to CIP minimum dimensions for .303 British barrels. The CIP dimensions are: minimum bore 7.70mm = .303", minimum groove 7.98mm = .314". If a .303 British marked and chambered barrel, even a custom one, does not meet these minimum diameters it will be rejected by the proofhouse, be it Liege, Suhl, Ferlach or London, and be sent back to the maker.
BTW, the maximum bullet diameter is 7.92mm = .312".
This loose fit, by todays standards, between minimum groove and maximum bullet diameter is a common problem not only with the .303, but with many others of the early smokeless miltary cartridges, f.i. the 6.5x53R Mannlicher, 6.5x54 M-Sch. At the time the dimensions were set, bullets were long and heavy with thin jackets, smokeless rifle powders were relatively fast burning. To keep pressures at bay, cartridge/ barrel designers often depended on the long, round nose bullets to slug up on firing.