Impervious to bullets or other high velocity projectiles. On a personal note, when I worked for Continental Telephone Company (CONTEL) in the 1980s, the central office (CO) in Espanola, New Mexico, was a regular target for disgruntled customers or folks just out for a little target practice. (We never were 100 percent sure. Espanola was a pretty wild place.) The CO technicians regularly had to dig rifle slugs out of the analog step-by-step (SxS) CO switch. With relatively minor repairs, that old electromechanical switch could be restored to full operational status, which attests to the ruggedness of the technology.The CO wasn't bulletproof, but was close to it.When we installed a new, and much more sensitive, electronic common control (ECC) digital switch, we had to bulletproof the CO with an armored door. See also aramid.