Here is something that I have been using for a while and is now very effective with the new machine guns. The basic bunker is a small bunker consisting of 7 blocks and can be build while the enemy is reloading. Build it using this diagram with the front pointed away from you. It will allow okay cover quickly.
Diagram: http://i1228.photobucket.com/albums/ee443/drewgon13/Blockbunker-basicform.png

I will post later directions on how to turn your basic bunker into a Normal bunker, an Advanced bunker, and so on until you have an entire fortress built.