By the mid-16th century in major manufacturing hubs like Graz, Austria, armorers routinely subjected breastplates to quality control tests using firearms. A pistol or light arquebus was fired directly at the completed piece at a set distance; if the ball only left a dent without breaching the steel, the plate was certified as 'proof.' The resulting indentation, known as the 'bullet-mark,' was deliberately left unpolished as a visible guarantee of life-saving protection for the prospective buyer.

The concept of armor being 'of proof' passed directly into the literary lexicon of Early Modern English. This technical jargon was quickly seized upon by metaphysical poets and playwrights, including William Shakespeare, who frequently used 'proof' as a metaphor for structural emotional resilience and moral fortitude.