I have 38 plastic bullets and cases. They are "loaded" with a primer and I find them great for plinking in my garage. I believe, after having to dig one out of the drywall after it had passed through 2 layers of cardboard, the box of crumpled paper and out the back of the box that these would do very well to discourage a dog/cat or other small animal from my backyard. I have since upgraded my target and backstop.....
They also work very well in assisting with bringing someone who has never fired a gun to the point where you can take them onto the range without much of the fears they may otherwise have.
Bought them from Midway USA and they are made by SPEER.
Just what Jason said. The bullet is actually molded as part of the case, and tears away upon firing. Neat stuff, lots of fun. The seller isn't kidding about lethality, at 25 yards they will compleatly shatter 4x4 wooden blocks. They won't function in self loaders, but work great in boltguns.
There is a guy who has a bunch of military parts the the local Houston gun shows, Martin (I think), that usually has them. I got a box a while back, thinking it would be fun to light some off in the house for practice. Dang they are loud, LOL. Not like the Speer plastic training bullets in noise level, but similar in concept.