The name is slightly misleading. Shotgunning a beer in no way involves firearms of any sort. WARNING: NEVER MIX ALCOHOL AND FIREARMS! EVER! Shotgunning a beer involves making a puncture at the bottom of the can, toward the edge, and cutting it out to a hole approximately the same size as the mouth on the top of the can. The puncture is made first before the tap is popped. Two reasons. One, the pressure makes poking the can much easier rather than it just buckling under the blade. Two, it is much less messy. As long as the bottom is facing up and away and the hole is poked in the side facing you, all that will happen is the excess C02 in the can will bleed out, and there will be no spillage. Once the hole is there, the tricky part comes. one must place their mouth over the hole, creating a good seal. Then simultaneously lift the can to standing position and pop the top and slam away. The term shotgunning comes from it just blasting into you. Just sort of all hits you at once.

The above image illustrates where to make the puncture. With the can pressurized it can even be accomplished with just a thumb at times. Below is an individual who is tilting the can too much and spilling. not the ideal shotgun position.