To make proof against bullets.
To make resistant to failure.
We have to bulletproof this program before we let the users at it; check every input, catch every possible flaw...it must not fail in use.
(of a material) Capable of withstanding a direct shot by a bullet fired from a gun.
A bulletproof window.
Reliable, infallible, sturdy or error-tolerant.
(usually of an idea or concept) Unbreakable, very tough.