Basically.... He used layers. Armored vehicles are defended in the same way against RPG rockets and high-caliber anti-tank weapons. M-1 Abrams armor is composed of layers of plate steel, ceramics, glass, plastics, or air, that fragment on impact when hit by a rocket. Theoretically, composite armor could absorb a shaped-charge explosion with 6" of layers that 12" of solid AND slanted plate/block steel couldn't stop.
I read an article were a 60 grain 22lr round penetrated 16-24 inches of ballistic gel at 200 meters. Article was titled lethality of the .22 LR (field test).
Are phone books the only thing being used to experiment? What about a box of dirt? I wonder if 12"x12" would be enough. May need to try it out this weekend.