Firearms Expert Reacts to DOOM Franchise Guns | EXP (2025)

Gotta address the lore after roughly 17:00 and straighten things up for you. So DOOM 1/2/64 all came first, where Doomguy was a regular, human Marine and there was nothing special about any of his weapons. At the end of 64 he stayed in Hell to essentially try to stop invasions from happening from the inside.
Then, as shown in Eternal's midgame flashbacks, Hell invaded the planet Argent D'Nur in a different universe far, far further back in the timeline (literally millions of years back) where human civilization originated from (at least in this new universe) and Doomguy stumbled out of Hell onto their world nearly dead and raving mad. Long story short, he was so impressive that they let him join their elite warrior order, the Night Sentinels, and after years of war with Hell he was plugged into the Divinity Machine, which turned him immortal and made him the Slayer.
Where this relates to the weapons is that he had his DOOM 64 weapons on him (at bare minimum he still had his Shotgun and SSG on him) and the Sentinels studied those weapons, reinforcing the very same original Shotgun with their sci-fantasy special materials, and completely from scratch forged a new SSG based on the original design. So starting in Dark Ages, his Shotgun is the reforged, upgraded version of his original Shotgun, and his SSG is a Sentinel-made custom piece forged for his use. From there lots of shit happened, lots of demons slaughtered with that SSG in particular, and then he got trapped in Hell and remained there for "eons" (so, millions of years) and probably still had that same gun on him until he inevitably got separated from it over the course of those "eons".
Samuel Hayden, the head of the UAC in the new universe, is actually the very same Seraphim who chose to ascend Doomguy into the Slayer, and so he knew all about his arsenal and, when he set up this new universe's UAC, he armed his elite guards with SSGs based on that same design. And in Eternal, the Slayer tracks down the original, Sentinel-made SSG given to him after he joined the Night Sentinels, and reclaims it. So no, that SSG was not a precursor to the weapons he used in DOOM 1/2/64; it didn't exist yet at that point. However, he did wield it for millions of years AFTER that, hence it being a "historic relic".