How to change GUI for gun magazines/clips

[draslin]

I can't seem to find any tutorials specific to modifying GUI elements. I've also dug through some of the Lua code trying to find something obvious but nothing has caught my eye as obvious. 

 

I'd like to see if I can modify firearm magazines to show remaining ammo in a way similar to how water bottles show how full they are. This would be far more convenient than manually looking at every individual magazine for ammo count. 

 

Can someone tell me what I need to look for in the Lua code or alternately direct me to a tutorial on the subject?