7.62x39mm (Russian) won't fire in a 7.62x51mm (NATO) rifle because the Russian round is narrower and will slip into the chamber too deep to fire. And conversel

7.62x39mm (Russian) won't fire in a 7.62x51mm (NATO) rifle because the Russian round is narrower and will slip into the chamber too deep to fire. And conversely, the NATO round is too big (length and wide) to fire in the Russian chamber. 5.54x39mm (Russian) won't fire in a 5.56x45mm (NATO) rifle because it's a slightly wider case and won't fit in a NATO chamber. The NATO round is too long to close the chamber on the Russian rifle.