The purpose of the .775 overbored barrel on the Moss 835 was to give better patterns vs the 12GA standard .729 bore on most shotguns at the time to near 10GA performance. The .775 bore when choked correctly, typically will shoot tighter patterns vs a .729 bore barrel that is also choked correctly due to the fact that the larger bore would not squeeze the shot as tightly as the .729 bore and thus less deformed pellets when shooting lead loads would mean improve pattern efficiency from the larger bore. Typically speaking the larger the shot size used in both of these barrels the even more pronounced the pattern advantage would be in favor of the .775 bore over the .729 bore. This explains why the Moss 835's have a tendency to shoot better 10" and 20" numbers with most turkey loads vs the other 12GA shotguns that have the smaller bore barrels. Now remember there are some shotgun manufacturers that don't use the standard .729 bore dimension on their barrels. Benelli shotguns use a .723 bore and Browning and Winchester guns that use invector plus chokes will have a .742 bore. But typically speaking the larger bore on a shotgun the better it will tend to shoot most shot sizes.
And to answer your last question, 10GA shells are simply put harder to find in most stores. Most Walmarts will have plenty of 3.5" 12GA turkey loads but go and try to find some 10GA shells and you will get the picture. And 12GA 3.5" shells are less expensive to shoot than 10GA shells.
