There are plugins already out there with guns, they just use things like books and other useless things for the gun. The only way to get a gun plugin with a gun look is through Spout. There are no plugins like that yet, though. I don't know how to code Java, otherwise I would make your plugin.
It has other features too! I did not make it though so whoever did gets credit! Well this plugin basically has a command where you type it and a stick turns into an arrow shooter! there is a grenade command where eggs turn into grenades, snowballs turn into smokebombs, airstrikes, miniguns that enhance a stick to shoot 5 arrows and is automatic, there is also a team command and an automatic stick shooter gun. There are maybe more commands on the page too! CHECK IT OUT!