Brass cases are more "springy" in that they will expand during firing and then retract to their original size as the pressure goes down, making extraction easier. Steel expands and then doesn't bound back as much, so it can get stuck more easily. This all depends heavily on the cartridge design.
7.62x39 has sloped cartridge walls (which is why the magazine is shaped like a banana) so it will extract just fine regardless. It's intended to be used with lacquered steel cases with no problem, even if the chamber gets super hot or dirty.
The main downside to the shape of 7.62x39 is that it massively increases bolt thrust for a given amount of pressure. The expansion of the cartridge against the walls exerts backwards thrust, potentially beating up the bolt and the lugs. The wide base doesn't help much either, especially in AR-15s (which don't have enough real estate to reliably deal with wide base cartridges- making enough room for 7.62x39 on the bolt face thins out the lugs too much).
The ejected bit.
Brass cases are more "springy" in that they will expand during firing and then retract to their original size as the pressure goes down, making extraction easier. Steel expands and then doesn't bound back as much, so it can get stuck more easily. This all depends heavily on the cartridge design.
7.62x39 has sloped cartridge walls (which is why the magazine is shaped like a banana) so it will extract just fine regardless. It's intended to be used with lacquered steel cases with no problem, even if the chamber gets super hot or dirty.
The main downside to the shape of 7.62x39 is that it massively increases bolt thrust for a given amount of pressure. The expansion of the cartridge against the walls exerts backwards thrust, potentially beating up the bolt and the lugs. The wide base doesn't help much either, especially in AR-15s (which don't have enough real estate to reliably deal with wide base cartridges- making enough room for 7.62x39 on the bolt face thins out the lugs too much).