Other key words here are "brass" and "steel".
As has been pointed out, steel cases are difficult to reload and rough on your dies. I sometimes make up a few steel .223s for keychains. I can do that because I have the dies.... and I don't have a rifle!
As to brass cases, there are brass 7.62x39 around. Many of them are Berdan also, but these CAN be altered to use standard Boxer primers. It is a nuisance, but it's definitely cheaper than buying fresh brass. In other calibres, especially some of the hard-to-get ones, it is not only possible but darned near mandatory to reload the Berdan brass. In many of these cases, it is pracical to re-form the casings to accept Boxer primers.... which are far more practical, anyway, not to mention cheaper AND easily available.