Let's get this done!
If you're going to make a gun so annoying to listen to when 12 people are shooting it at one time, might as well make it funny!
Thus TOK TOK TOK becomes DOOOOFizl DOOOOFizl DOOOOFizl.
EDIT: Some other ideas from thread:
WAKKA WAKKA WAKKA
BOOF
LIKE, ZOINKS!
Human Goat screams: https://www.youtube.com/watch?v=PpccpglnNf0
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What about "pew pew pew"?