[thackney]

Steel cased means the steel is not in the bullet. It is the “case” that holds the bullet, primer and powder.

I’ve shot it as well. The bimetal bullet is lead jacketed with copper.

[Sender]

A lot of Russian ammo is steel core with a copper jacket. Probably because it’s cheaper for them.

[Gaffer]

Steel-jacketed bullets[edit] In addition to using steel casing, certain types of Wolf rifle cartridges use steel jacketed bullets, which are often copper-plated and cosmetically similar to standard copper-jacketed bullets. The copper exterior of the bullet is approximately .005 inch thick, with an underlying steel jacket of about 1/32 inch thick. This type of ammunition is labeled "bimetal". Indoor shooting ranges, which use backstops often constructed of steel, have accordingly widely prohibited steel-jacketed and bimetal ammunition to prevent shooters from damaging their backstops

From a Google on it. The range I shoot at banned them because of the underlying steel jacket's potential to cause excessive wear on their backstop.