[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.

[thackney]

Thanks, so not a steel core, but a thin steel jacket.