This mod edits the spawning loadout file for the Heavy Assault Rifle so you spawn with a Chaingun instead. It fully works and collecting ammo will fill up the chaingun as well. Secondary fire/weapon mod makes the chaingun spin up and fire faster. Install with DOOMExtract.
DOOMExtract is a command line operated file that can perform operations on the resources file for DOOM, including extracting it, and repacking mods into it. To install extract this to the "/base" directory and drag "gameresources.index" onto the program. This works in all current available builds of DOOM.
To install mods, run the install.bat or edit it to the mod install folder you wish to use.