I will actually talk about Classic Doomguy's Weapons now:
The Pistol is based on the Beretta 92, it fires 9x19 mm parabellum bullets at 381 m/s, giving it a muzzle energy of 540.725 joules\
The Shotgun was based on the TootsietToy Dakota Popgun
Which looks similar to the Winchester 97 "Trench Gun"
This specific shotgun he uses has 7 pellets in its shell, shotguns of this kind tend to go around 1300 fps. For the pellet size well:
The most common type of
ammunition used in combat shotguns, whether for military or law enforcement purposes, is
buckshot, typically a 70 mm (23⁄4 inch) 12-gauge shell loaded with 9 hardened 00 buckshot, with a diameter of about 8.4 mm (.33 inch)
~Wikipedia
Given they're made of lead that's 3.5037386 grams per pellet, or 275.054 joules per pellet. Times 7 and that's 1925.378 joules assuming a 1300 fps muzzle velocity.
However shotguns don't go by penetrative power, they go by its momentum that's 1.3883 kilograms * meters/per second for a single pellet, and 9.7181 kg.m/s for a direct blast of all pellets
The Chaingun came from the TootsieToys "Ol' Painless" Gun Toy, which is based on the Minigun used in Predator. So it's a minigun.
The M134 Minigun fires 7.62×51mm NATO (10 gram) bullets at 850 m/s. That is a muzzle energy of 3612.5 joules per bullet. Ingame it fires 525 rounds per minute, but (as I will explain later) in real time that is ~4667 rounds per minute, which fits cozily with the RL minigun's firerate of 2000-6000 rpm.
The Rocket Launcher isn't based on any RPG in particular but it looks closest to the M47 Dragon:
The M47 Dragon fires a warhead with 3.586 pounds of octol
To find the energy of this amount of octol, we will go by its relative effectiveness. It has an RE factor of 1.54, meaning that Octol is 1.54x more powerful than TNT.
3.586 pounds * 1.54 = 5.52244 pounds
5.52244 pounds of tnt = 11,552,944.48 joules
That's roughly how powerful the explosion is going by a real life estimate. Ingame, the Rocket has a much smaller explosion than that, with a blast radius of 128 map units. How big is that?
The M47 Dragon has a rocket velocity of 200 m/s, but in game it goes at 700 map units per second, or 22.5 meters per second. We can actually derive a timescale from this:
700 map units per second
200 meters per second
56 map units = 1.83 meters
700 mu/s = 22.5 m/s
200/22.5 = 8.889
So Doom time takes place 8.889x faster than real time.
The BFG 9000 or the Big Fucking Gun is well... The Big Fucking Gun. It can kill a Spider Mastermind in one hit and has a massive range thanks to its tracers. Each individual tracer can gib a zombieman
0.457 seconds/8.889 = 51.4118574 milliseconds
1024 pixels = 33.46 meters
33.46 meters/51.4118574 milliseconds = 650.822625 m/s