I've seen reports of bullets ricochet off of everything from glass, brick, dirt, even water. It all depends on so many variables including speed, angel, air density, temperature, etc. The only thing I can think off that a round would not ricochet off of is a human body (maybe a skull). Warning shots are NEVER a good idea considering the legal effects of a stray round going somewhere it shouldn't. I would hate to think of firing a warning shot only to have it bounce off of something and strike me. Then the BG gets my money, tv, and even worse my gun. The only time a warning shot would make any sense to me is if it's meant to scare off a large dangerous animal such as a bear or something that would only get really ****** if you shot it with a 9mm.
A hit with a .45 ACP beats a hit with a .22 LR everytime. A hit with a .22 LR beats a miss with a .50 BMG everytime.