The DOOM slayers weapons cannot be regular weapons, the barrels would have worn thin enough to be used as sheet metal if that was the case. If you play the games you never see any maintenance, cleaning or repair happen to the weapons. The closest you get is when you upgrade the weapons.
Either the weapons manufacturers in the Doom verse have cracked some sort of totally amazing metal that can withstand friction for literally hundreds of thousands of shots or the slayer reinforces the weapons he picks up with some of the energy he steals from demons.
If there is a new alloy for use in weapons I would imagine that most companies would use the added capability to increase chamber pressure and projectile velocity and thus wear out the barrel in about as short a time period as before or use the stronger metal to decrease cost of manufacturing by using very little of it and simply using lead ballast near the shoulder stock to decrease recoil, would make for a jittery weapon for most people though.
Consider the time the games were made. Also consider that modern shooters
still don't include weapon degradation yet you are suppose to be using bog standard firearms. Likely ones that weren't even properly maintained when you acquire them.
The timeline for Doom goes as follows:
Doom: Doomguy is a marine being disciplined for insubordination, he's left behind with only a single pistol to 'guard the exit' when his squad is sent to deal with an emergency. They all die. In this game his weapons are bog standard guns, for the most part. The plasma gun and BFG are pretty much the only ranged weapons that aren't normal firearms that use normal ammunition. He has to find ammo stashes to restock on bullets. The chainsaw is an exception in that it never runs out of fuel, but also doesn't insta-gib almost everything. It's basically an improved melee weapon. Speaking of which, if you run out of ammo you can use brass knuckles. Not the best idea, but it shows that none of the demons are immune to mundane weapons. The game ends with him stepping out of a portal from hell, and finding him self outside Sidney. This is where he learns his pet rabbit Daisy has been killed by demons.
Doom 2: Probably follows someone else. In this game, which is suppose to take place right after the events of Doom, you play as a marine who also had been stationed on Mars, who has just arrived via drop ship from Mars following the events of Doom. He's one of the few survivors of the incident. Once again your weapons are bog standard kinetic weapons with a couple energy guns. Don't know if the chainsaw is in this game, but probably was.
Doom 64: This time the game follows the Doomguy from the original game, and has him returning to Mars when a new hell invasion begins. Once more he uses bog standard weapons, and needs to find mundane ammunition or he'll run out. Game ends with him closing the hell portal from the other side, trapping himself in Hell. He's still pissed about Daisy being killed, and has declared War to the Knife, Knife to the Hilt on demons.
Doom 3 is an alternate timeline that retells the events of Doom 1 while expanding the story a great deal
Doom 2016/Doom Eternal: The original Doomguy, now known in hell as The Doomslayer is still alive and still a major threat to demons. In yet another timeline he's found trapped in a sarcophagus after the demons eventually managed to capture him. At some point between Doom 64 and Doom 2016 he was given the ability to absorb the essence of demons to restore his own health and vitality. In this specific timeline the weapons he manages to rearm himself use solidified argent energy (or hell energy) instead of normal bullets. This was changed to explain how he can restock on ammo by taking enemies out with a chainsaw. It's also why you can restock all ammo types from a single (glowing blue) ammo stash instead of having to find ammo stashes of specific types of ammunition. Also at some point between the two games his armor was upgraded from standard (and honestly kind of flimsy) Marine Armor to his Praetor Armor seen in Doom 2016 and Doom Eternal.
According to Doom Eternal, Argent Energy is merely refined hell energy. It's refined via the torture of human souls in Hell.