...Thank you all, it's very interesting. The heavy Parkers 20s appear to ba made as deliberate long-range weapons, and go even further in this respect than the Ivashcntsev-inspired heavy 20s which were made as all-around game guns.
...
On the practical side, the Sokol brand smokeless gunpowder, the most common one before 1917 and almost the only one widely available in the Soviet times, is a pretty capricious chemical - it really needs high chamber pressure to work properly and functions in small bores better than in big bores. Especially if you want to use light loads, 1 ounce or less, in the 12 gauge - hopeless. So for someone who is limited to Sokol and has to use both regular 12 gauge loads and light 3\4 oz loads, the heavy 20 is about the only way to go.
It would seem, then, that the most logical explanation for the Russian heavy 20 - shooting a 12ga load through a 20ga - is that powder chemistry and physics were driving gun design to accommodate the powder's characteristics. For whatever reason, powders better suited to the 12 were not available in Russia, and customers desired the performance of 12 ga. So ... make a gun that gives that performance while working with the powders that were available.
An entirely different reason than the American desire to reach out and touch waterfowl at long ranges, but yielding a similar solution.
