I wouldn't know where to even start to do, so I ask if someone else could do it.

It's a version of the game's legally distinct beretta pistols, https://static.wikia.nocookie.net/hitman/images/2/21/"Rude_Ruby"_HITMAN2021.jpeg/revision/latest?cb=20210306020016 so if someone could port it over, it would work the best on the Bernetti 9 if someone can pull or recreate the textures from Hitman and put them into Payday 2.