Hardcore Difficulty

Version: 1.2.1
Sleeping now does not recover health core or dead eye core

All cores completely drained upon death while not on a mission
Damage received: 400%
Damage delivered: 120%
Health regeneration: 5%
More to come!

Installation

Download RAGE Plugin Hook for RDR2
Extract the contents of RPH into your game directory, where RDR2.exe is
Extract the contents of the mod into the game directory (C#Plugins should be in the same directory as RDR2.exe)
Launch RDR2
Run RAGEPluginHook.exe from the game directory

If the mod loaded successfully, you will see it notified in the top left corner of the game.
You can hit F2 to toggle the mod on and off.

Unfortunately, RAGE Plugin Hook is not (YET) compatible with other script hooks.
In order for the hook to work, make sure you have overlays such as FRAPS and RivaTuner disabled.

Updating

To update the mod, overwrite the HardcoreDiff.dll in C#Plugins.
To update the RAGE Plugin Hook, choose the update option next to the download button on the website and drag the contents into your game directory.

Planned Features

Health and Stamina Cores deplete faster
Lowered maximum ammo
No bullet trails
Ammo drops must be looted from bodies, instead of simply walking over them

[source]

Credits:

Vesdii and Unwound