I found this out the hard way when I took my new CZ-82 to the indoor range with the only 9x18mm ammo I could get: Norinco FMJ, Brown Bear FMJ and Silver Bear JHP (the Bear ammo stated "bimetal" on the box). When I entered, the sign on the door said: "No Wolf Ammo." I thought about it for a second and decided, 'I don't have any Wolf ammo, so I should be fine.' After my bullets caused some sparking off the steel backstops and some suspicious looks by the range staff, the sign at that range now reads: "No Bimetal Ammo." In the past this issue has been mostly isolated to common Russian calibers such as 9x18mm and 7.62x25mm. Today, however, more bimetal alternatives are being imported by manufacturers such as Wolf, Herter's, Tulammo, Ulyanovsk, and Barnaul (which makes Brown Bear and Silver Bear) to name a few.
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OK, it's really not that shocking. The bullets turned out to be lead-cored, but steel jacketed, with a very thin copper coating over the steel. So, the term bimetal refers only to the jacket and not the bullet as a whole. How thin is this copper coating, you ask? Very thin. I measured five different bimetal rounds and the outer copper coating was only about 20-30 micrometers (abbreviated µm). How small is that? Well, there are 1,000µm in a millimeter and the hairs on your head range from around 50-100µm thick