There's a YouTube channel that I watch called SmarterEveryDay, in one of his more recent videos he used a slow motion camera to see how a bullet would effect a Prince Rupert drop. Before I talk about the video I will first explain what a PR drop is. How they are made is some molten glass is dropped into some cold water, creating an incredibly strong price of glass. However everything has a weakness, in this case it is the tail of the glass piece which is incredibly fragile. The hardness of the glass comes from the rapid cooling creating a bulb that has a cold exterior that pulls inward on the hot interior which pushes out. These forces equal to something that is bullet proof.
And here is another video of his showing the properties of the PR drop.