How do you get an automatic shotgun in Mafia Wars?

When you do a mission with Irish to get the machine gun, there is the hill you have to go down and kill people. There is a house at the bottom of the hill where Irish said the machine gun is in. There is a guy hiding inside. When you walk close enough to the house, he jumps out with a double barreled shotgun. You can kill him and take it. But if you passed that mission and you don't have it, in Mexico, the people walk around with them.