What is difference between 7.62x38 and 7.62x39 ammo?

The 7.62x39 round is an intermediate rifle round used in rifles like the SKS and more famously in the AK-47 & its variants. The 7.62x38 (technically called the 7.62x38R) is a pistol round used almost exclusively by the M1895 Nagant revolver.

The 7.62x38 is a very distinct round with brass that extends past the bullet so that it can more effectively complete the gas seal which is a major feature of the M1895. The casing is also strait when using surplus ammo, and has a crimp at the end of recently produced ammo. Meanwhile the 7.62x39 -being a rifle round- is much thicker at the base than it is at the bullet allowing for much more powder.

These rounds are NOT interchangeable in in any way. Only because rounds are measured to the end of the brass, the 7.62x38 cartridge is near the the length of the 7.62x39. Without the extra brass the 7.62x38 is more along the lines of a 7.62x25. I hope this answers your question without being technical or talking down.