@Batter
Twilight's script is overriding the weapon icon? Must be an out of date script? I used the ones from the latest updates, I believe. You can subscribe to my scriptless version if you'd like.

i found the issue. twilight's upload has a script file inside wich overwrites your script

i tried all of the above + placing the file in the addon folder instead of the workshop one and the hud icon seems to simply not work. it's using the default ssg hud icon

@Batter
1. Check and make sure you are subscribed.
1a. If you are subscribed while the game is open, exit and restart the game.
2. Verify the file is in your "\steamapps\common\Left 4 Dead 2\left4dead2\addons\workshop". The file is/should be named 2282204283.vpk.
3. Check your "Add-ons" section from the main menu in Left 4 Dead 2. Be sure HK416-C HUD Icon (SG552) is listed there and has a check mark next to it.
4. Ensure the text of this and any other addons are not Red, as that indicates a conflict.

If 1-4 fail, unsubscribe, resubscribe, verify game cache. I've just downloaded and verified this file myself, so it is indeed working.

it's not working