how to access a specific instance of a script

hello. i have a script that a button uses to start a function on my gun script that buys ammo. problem is that i have multiple instances of my gunscript on the same camera so when i use sendmessage from the button to the camera it adds ammo only to one script. so i need to access a specific instance of the gunscript so it adds ammo to the weapon the player is using.

Assign it from the inspector. By making a public variable and than dragging the script you need into it.
or.
Tag the instance you need and look up the instance by tag.