The experiment was performed by researchers from NC State, led by Afsaneh Rabiei. It saw a M2 armour-piercing projectile—0.3 inches in diametre—being fired at a lump of metal foam. Rabiei explained to PhysOrg what happened:
“We could stop the bullet at a total thickness of less than an inch, while the indentation on the back was less than 8 millimeters. To put that in context, the National Institute of Justice standard allows up to 44 millimeters indentation in the back of an armor.”
Which is just, wow. These metal foams can be made in many different ways. Some are manufactured by bubbling gas through molten metal, while others cast metallic alloy around hollow metal spheres to provide voids.