Gents;
My Ruger 7.62x39 is a Model 77 bolt action which I bought specifically for use with cast bullets because of the reduced case-capacity. Its barrel slugs at .3105", tested several times to make sure. It is NOT the same as a couple of other Ruger 77s we have here in .30 caliber, which slug .3085" and .308" even, respectively (a .308 winchester and a .30-06). BTW, the 7.62x39 M77 shoots cast bullets very well indeed.
My experience with the .303 British spans five decades and MANY rifles. Their barrel dimensions vary all over the map, which is not surprising considering their manufacture spans at least seventy years and half-a-dozen countries, and throwing in some deadly-serious wartime emergencies as well.
It's not uncommon to find Lee Enfields with groove diameters as large as .315" or even more. The NOMINAL diameter at the top of the lands is supposed to be .303", but this also is a highly-variable figure. On top of all this, it's virtually impossible to accurately-measure a five-groove barrel without equipment that most of don't have.
To say that a .303 British has such-and-such a groove or bore diameter as if it were cast in stone is inaccurate, to say the least. Nominal dimensions are one thing, but what we find in individual rifles can be a LOOOOONG way from "the ideal". I still love 'em, though...wonderful pieces of history, and still great shooting entertainment.