Normally it’s shown as a series of silver barrels bound together toward the muzzles, with hand-drawn lighting effects showing that it’s made of reflective metal.
Modder TechIguess explains on the ModDB page: “Yup, I literally just took Doom 2 and redrew the weapon models to make them look worse.” If you were to judge the mod by how well it accomplishes the goals the modder set out for it, it’s a smashing success: all these guns look extremely bad. ‘Doom 2 but the guns look bad’ doesn’t touch anything else in Doom 2, it just makes the guns look bad. There’s a new mod out called ‘Doom 2 but the guns look bad,’ which replaces all the weapon artwork in Doom 2 with a version that looks like it was made in Microsoft Paint, probably because it was. Other modders have other goals in mind, however. Doom 2 has been out for a long time, and the Doom modding community is still endlessly creative with finding ways to add to, change, or completely remake the classic series of gory FPS games.