Well it is in one way. The 123-grain 7,62x39mm bullet can be loaded in the 7,62x53r/7,62x54r full-length cartridge to make a more comfortable and less brutally recoiling load with less flash at night; particularly in Mosin Nagant M38 and M44 carbines.
Just as easily the same .311-.312 bullet can be used in loads for the .303 British, 7,65 Mauser *Argentine* [and Belgian] and 7,7mm Jap, among others. And the .312 hollowpoint bullets meant for the H&R .32 magnum pistol can be loaded into the above cartridges as well; all in the appropriate cartridge cases, of course. There've also been adapters made to allow the use of the 7,62x25mm Tokarev or 7,65 Mauser pistol cartridge in 7,62x54r chambered rifles and some 7,62x39mm weapons for single-shot indoor practice at very reduced ranges; 50 feet or so.
But as a complete loaded round, you're quite correct; they're not at all interchangable.
-archy-/-