It is my understanding that a dealer may only have one of a particular post 86 sample. One G36, one galil, one M249... Each one must have a law enforcement demo letter to go with it. Something like "We of department such and such would like a demo of the M249 machinegun so we can decide if we will buy one."
-I am not a dealer. That is just my understanding of how post samples are regulated. I could be wrong.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.