New material developed to stop .50 cal rounds . Lighter
https://www.military.com/video/new-metal-foam-can-stop-50-caliber-rounds
https://www.military.com/video/new-metal-foam-can-stop-50-caliber-rounds
Makes sense, the air pockets inside the foam, absorbs the force of the impact.
Sort of like shooting a pistol round into a snow bank. It doesn't penetrate very far.