You can't re-chamber a .303 to .30-30. The body of the .303 is bigger and the reamer would mostly churn against air instead of cut steel. To do so would require a fair amount of barrel setback, and since Jeff said the sight dovetail remains at the original prescribed distance, that didn't happen.
Another scenario would be someone bought, begged, borrowed, or stole an unchambered .303 barrel blank from Savage/Marlin and fitted/chambered it as a .30-30 on his (or a customer's) M1895, or else it indeed was done at the factory.
