The idea is to drain the can as quickly as possible, which isn’t possible if there’s only one hole as you have this constant back-and-forth of liquid flowing out and air flowing back in, which interrupts the flow of liquid out.
Shotgunning relies on punching a hole in the can (to drink from), and then opening the regular tab, meaning you get one hole where liquid can flow out, and one hole where air can flow back in.
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