I have a story about M4 buttstocks coming into contact with other objects. A few friends of mine and I were at an abandoned house in the middle of nowhere, shooting shiit up. There was still some glass in some of the windows, and I decided to smash them out with the buttstock of my M4. Big mistake. The thing literally cracked in half at the first blow, and I obviously had to buy another one.
The AK, on the other hand...
If I had to smack somebody with a stock, I'd have to take one of those older military bolt-action rifles like the Mauser or Mosin-Nagant. An M14 stock would do nicely too. Anything that's made out of wood, thick plastic or metal and is in one piece!