Yeah, it's in the game's code, but it's there as an extra "unfair" advantage. You get a new gun by buying it or finding it or reaching a certain point in the story. Cheat codes are pretty much a free lunch. I don't see anything wrong with using them, but it's still playing outside of the standard game rules.
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"God Hand is the ultimate expression of the joy of humanity, specifically the punching part of the joy of humanity."-Shigeru Miyamoto