Since someone brought this up.
People with their backs to the door should hit the ground to give those facing the door a clear field of fire.
Note that it is bad etiquette to accidentally shoot your poker buddy in the face (unless you are a former Vice President).
Seriously, a good steel door in a good steel frame which is LOCKED and a couple security cameras should either completely deter an invasion or slow it to the point that defensive precautions (funnel of death) can be enacted.
Remember kids, proper prior planning prevents piss-poor performance.