To make proof against bullets.
"The security team installed a special glass partition to make the president's office bulletproof during his speech."
without flaws or loopholes
"an ironclad contract"
"a watertight alibi"
"a bulletproof argument"
Capable of withstanding a direct shot by a bullet fired from a gun.
"The armored car rolled past the protesters completely unscathed, its reinforced glass proving truly bulletproof against their attempts to breach it."