Ok. Go into the first block of the mission. Have your machine gun clicked so you don't have to hold on to the button and press the button to change weapons. Then while still on the weapon palette, go onto the monomate or any healing object and stay on it. While on the healing object, leave the mission and go into the city or where ever you go into the mission. Then let go of the palette button and go into the mission again. Enter the mission and hold the attack button. Then press the palette button as fast as you can and Eureka! There is the machine gun glitch!*

*Note: You have to be online and on Xbox Live.