Shotgunning a beer is just like draining a full barrel of water (mind you a very small barrel, but conceptually the same). This brings in a special rule in fluid dynamics, the Torricelli Law. This equation relates the flow for a draining tank to the level of the fluid still in the tank. The Equation has MANY aspects to is but the basic set up is basically v=(2gh)^.5. v is the velocity of the fluid leaving the tank, g is the force of gravity (9.81 m/s^2) and h is the hight of the fluid in the tank. So how does this apply? Quite simple, the flow through the bottom hole (the drinking hole) can be calculated along with the flow through the normal spout. Add in the air pressure from the top spout and the force of suction from the drinker, and the result clearly displays that shotgunning is a much faster way to consume beer.

The following links go FAR more in depth on the principle, for a better understanding of how  this concept works go to:
http://www.ace.gatech.edu/experiments2/2413/torricelli/fall02/TorricelliLaw/analysis.htm
http://gershwin.ens.fr/vdaniel/Doc-Locale/Cours-Mirrored/Methodes-Maths/white/deqn/a1/torricelli/torricelli.html