Ball 7.62x39 varies, some mild steel core, some lead.

Generally, there is a lead sheath of varying thickness between the core and jacket, allowing impression of the lands into the bullet. However, some hard-core bullets are said to be VERY hard indeed and of course these bullets are known to increase wear on barrels as well.

Even so, one wonders how hard {really, difficult to engrave} such bullets would be compared to the Barnes or other monos.

What's a bummer is that it seems that every time we find an OSR'd barrel, measurements or other observation for those of us who have never seen it is, for some reason, not possible. IIRC, Mark Cash did post a pic of one back in antiquity, but I don't recall the details, nor do I remember if there were any dimensional changes noted.