If you can't measure it, it doesn't exist
When I see, "changed the game", I literally interpret it as that. What guns caused enough stir to cause a change to the game? Not "what guns do you ascribe some emotional/sentimental value to?" which is quite frankly a boring, vapid question.
Well, what comprises a game? The rules. The rules are the formation of the game. If you change the rules, you change the game.
For instance, in many places the SMG-60 got banned. I.e. it forced a modification to rules.
The Shocker turbo also forced the rules to start thinking about electro trigger criteria.
Reactive triggers -- Automag RT , etc. -- got banned.
The first semi-auto basically implicitly created the distinction between semi and pump-only style games.
First auto-trigger based gun probably prompted the first ban on auto-trigger per strict stock class rules.
And along those lines, I don't know what was the first gun to use CA, but it was immediately banned.
I fail to see how the Tippmann 98 actually "changed the game". Did the game change to accommodate the 98? Or did it change to exclude the 98? Or perhaps neither? I mean, I personally have a strong emotional reaction to the Tippmann 98, but that is completely independent of any changes to the game it had.
I also do not see “a gun” changing the game format from woods to speedball. People went from woods to speedball because they wanted to, not because their talking gun made them do it.