I have seen those inserts. They are shaped like the 7.62x54R case. You load the 7.62x39 into the insert, then chamber the insert like a cartridge. You fire, extract the insert, poke the empty case out with a rod, and reload. Tedious, but it would work after a fashion. I have not used them, so I don't know how well they stand up or if the insert expands under pressure.
Jim