I've found one of these little squares on the local range one time, somebody forgot they had it so I took advantage of the situation.
I found an aluminum can and placed it out on the target frame while placing the little explosive square on top of it. I shot it with my custom AR15 from a distance of about 25 yards and was surprised by the "boom" it set off, sounded about like an M80 when it went off. And it SHREDDED the aluminum can!
I managed to head out and buy a box of these things, got 8 of them for like 6 bucks. I need to head out to test these to see if they are similar in performance.
They use what looks to be a magnesium or aluminum powder inside of them and rely on being quickly saturated within the air while detonated by the shock of the impacting bullet, velocity counts here and the faster the better. I wasn't able to get one of them to work some time back while shooting with my Ruger MK II 22lr. pistol, the bullets punctured but didn't have the velocity to set the things off. There is about a 1inch diameter circle within the target where the "stuffing" is located, it would be highly recommended that you nail these things on the first shot so use something that you're pretty confident will drill the center on the first shot.