Shouldn't Rika remember that guns are useless in this game?

There are a couple of believable explanations here:
1) Her friend Mr. 9 is being a bad influence on her.
2) For whatever reason, she might be _intentionally_ playing Miss Wednesday as the gag character that she was in the source material before the truth about her was revealed at Whiskey Peak.

I mean, if you make them big enough they might do something. Right?

She's just roleplaying her character, i think.
OOC, she knows full well that it won't be enough, but it's alright, since she is not trying to succeed at killing laboon anyway.

Hey! There are several parts in this entire series where guns are useful. I can think of *two* people off the top of my head! I mean, yes, that is counting all of One Piece up to current manga chapters but still.

Pinkies out, never forget

Crap, I never noticed that.
Heh.

What does that even mean?

Pinkies out means you're trying to be like excessively/stereotypically fancy and/or proper.

Even if the Bazooka could harm it, i'm not sure killing something that big while you're still inside it and it' underwater would be a good idea.