The phrase "to stop a bullet" means to prevent a bullet from hitting and injuring someone by using a shield, barrier, or protective equipment. Full definition
I seem to remember a news item a few years back about someone who was shot but he had a bible in his pocket, which stopped the bullet. (nakedpastor.com)
While natural shells wouldn't stop a bullet, finding a way to create such nano - layers in military - grade ceramics could help build better shields, says Ortiz. (newscientist.com)
That's why the windows are 3.2 centimetres thick — enough to stop a bullet from an AK - 47. (moneysense.ca)