Out of just the mouth of the can beer has a fairly slow flow rate. The problem is no air is getting in to push the beer out, so as it flows a vacuum is created within the can slowing the pour. It is also what causes the gurgle at the mouth of the can. The flow of air into the can as slowed even more when a person drinks from it. One less intense method for drinking fast is to poke a hole just above the mouth of the can to allow for air flow that way, however, it more or less just gets rid of the gurgle and does not dramatically increase flow. Not to mention the can cannot go bottoms up with that method or it will cause a mess.

to really express the powers of shotgunning lets first look at a gas law.


Ideal Gas Law: PV=nRT
P= Pressure
V= Volume
n= number of moles of gas
R= The gas constant
T= Temperature

So for now lets call Temperature constant. Obviously the volume will also remain constant, as will R. So, what shotgunning does is increase the number of moles of air into the top of the can, which will in turn increase the pressure within the can, forcing the beer out through the bottom hole even faster. Not to mention the added suction of the drinker.