If a firearm has ammo, but the ammo is not chambered, the ammo icon does not appear. If you were then to chamber the round, then place it back on the ground the ammo icon appears. For this specific case I have used the repeater for a reference image, but have noticed it on many more.
Was not sure if this was intentional or not, but now I cant trust an "empty" firearm.
I think the icon is intended to show if there is a round in the chamber. It would still be nice to have an indicator to see if there are rounds inside the internal magazine, especially for players that play without the HUD.
Hello Bigcams.
As papersheet56 said, this icon is just intended to show if there is a round in the chamber.