From the derivation section for !’s pronunciation as “bang”:
Alternatively it could have come from comic books, where the words each character utters are shown in a "balloon" near that character's head. When one character shoots another, it is common to see a balloon pointing at the barrel of the gun to denote that the gun had been fired, not merely aimed. That balloon contained the word "!" -- hence, "!" == "Bang!"
I have read extensively through the history of comics, I make them myself, and I don’t think I have ever seen a depiction of a gun firing whose noise is delineated by a word balloon coming from the gun that simply says “!”. Usually the sound would be shown as a “sound effect”, which is just a large word floating in the air near the gun, not contained in a word balloon. And this word would usually be BANG!. With or without the !; it’s very common for sound effects to be unpunctuated.
Alternatively it could have come from comic books, where the words each character utters are shown in a "balloon" near that character's head. When one character shoots another, it is common to see a balloon pointing at the barrel of the gun to denote that the gun had been fired, not merely aimed. That balloon contained the word "!" -- hence, "!" == "Bang!"
I have read extensively through the history of comics, I make them myself, and I don’t think I have ever seen a depiction of a gun firing whose noise is delineated by a word balloon coming from the gun that simply says “!”. Usually the sound would be shown as a “sound effect”, which is just a large word floating in the air near the gun, not contained in a word balloon. And this word would usually be BANG!. With or without the !; it’s very common for sound effects to be unpunctuated.