

PROTON_EAC_RUNTIME="/home/username/.steam/debian-installation/steamapps/common/Proton EasyAntiCheat Runtime" %command%
Gets past EAC initailization. After that, it crashes on startup (white screen)
gamemoderun %command%

NOTE: DEFUNCT LISTING- Ironsight now works with Proton/EAC, but the correct listing for the game is a now separate one, under appID 783770.
Posting this here because it is the first search result for "Ironsight" and I don't want players who end up here thinking it's borked, because it works great. (I believe there may have been a rerelease or change in developer/publisher coming out of beta, resulting in the duplicate listing.)
This game no longer exists.
This game is no longer exists with appID #715220
It is fully playable. See here: https://www.protondb.com/app/783770
Only steps are to previously install Proton EasyAntiCheat Runtime and disable Steam's overlay.
You need first to go to your library, search "Proton EasyAntiCheat Runtime" and install it, if it is not previously installed.
If steam's overlay is enabled, the mouse and keyboard work in the menus, but NOT during the gameplay. I disabled steam's overlay and worked like a charm!

Developer must enable EAC for Linux
Tried Proton experimental as well. EAC stops even starting the game.
Still Borked
I made sure EAC runtime was installed and tried various proton version.

Gets past EAC and crashes
Devs need to enable EAC support for us to play it.
Shows loading bar animation then fails to start
Loading EAC and then crashes.

Il gioco si fa installare ma non si avvia
Ubuntu + rx 6800 xt driver mesa kisak
The game launcher pops up but disappear a few seconds after.
Le logiciel anticheat fait planter le démarrage du jeu

I was able to preform first time setup and EasyAntiCheat and then I was sent to the launcher and it closed using Proton 4.11-13.
proton 5.0-6 did not work at all.
The game launcher pops up but disappear a few seconds after.
The launcher appears blank for a few seconds before disappearing entirely.
Game shows a splash screen then crashes.

Launcher opens for a few seconds and then crashes
The game uses EAC, so you won't be able to play without a workaround or until a proper support for Linux comes along.