From Hitler moustaches to "sweat" in Mortal Kombat, Nintendo have a long history of over-reacting to violence in video games. So it shouldn't surprise you that, in 2009, you're still not allowed to call a Wii peripheral a "gun".
Speaking with Game Informer, Activision's Jeff Muench has said that when branding and promoting the amazing peripheral for the Wii version of Cabela's Big Game Hunter 2010, the company were prevented by "some obscure Nintendo regulations" from calling it a gun.
Instead, they had to call the large, gun-shaped peripheral made solely for use in a game where things are shot, a "firearm". Which means exactly the same thing.
Wonder why Nintendo went with "Zapper", then, when they could have gone with "Wii Hand Cannon"?
Hunt's On With A Pair Of New Cabela's Games [Game Informer]