(Hopefully I get this one right >.>)
With the "currentWeapon" sprite, you can have multiple frames, each one being a sprite of a different weapon, and set the animation speed to 0 so it'll never actually change frame on its own. Then you manually tell it which frame to go to based on the value of 'weapon', so if 1 means shotgun and the second frame of "currentWeapon" is the shotgun sprite, say this:
+ player: Value 'weapon' Equal to 1
-> currentWeapon: Set animation frame to 2
and so on and so forth.