Ok go to files\languagebank\english\gamebank\mygame\levelsetup
You'll notice the first several lines define the default huds. i.e. health, ammo,weapon, etc
What you'll have to do is write a script that displays the hud you'd like. You'll have to do an image for each round fired.
and have a line for each round fired. YOull also have to change the numeric hud for the ammo to display an image hud.
Look here I did a similar hud with health. I did it a little different I deleted most of the stock hud definition lines and added my own. I have eight huds each displayed when the health gets between a certain range. Whats cool is you can also add the setposteffect command when the player health gets say below 25.
http://forum.thegamecreators.com/?m=forum_view&t=201502&b=24