"It's exactly the same cartridge (the Soviets just didn't like the name)."
Soviet-era cartridges show some dimensional differences from German cartridges, and a number of references, including Cartridges of the World, show those variations.
Two theories are that they were simply manufacturing variations brough about by variations, wear, age, etc., in manufacturing equipment, or that they were redimensioned to better suit equipment that the Soviets had on hand.
That said, it's almost always the case that the two rounds are interchangeable.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.