I'm not a ballistics expert, but I don't understand why all of the ejected cartridges in the Michael Brown case were found on the street. It seems to me that based on Darren Wilson's testimony that the first shot, fired through the door was fired while Michael Brown had his body in the window. That makes it very unlikely, but not impossible that the ejected cartridge would be able to get into the street. The second shot was also supposed to have been fired in the car. I would have expected one or both spent cartridges to have stayed in the car.
The report from the crime lab says on page 3 that the cartridges were from "street in front of 2964 canfield" - see here
Maybe someone with some actual investigatory experience can comment?