A shield's purpose is to defend, or a bulletproof vest, or... Try using a gun for that and it ain't gonna work. You might say that a gun's purpose is to deter?

A shield's purpose is to defend, or a bulletproof vest, or... Try using a gun for that and it ain't gonna work. You might say that a gun's purpose is to deter? But the only way it can defend is by producing lethal force, so the defense can only be a potential secondary effect.