While 3D printing is usually used in the world of Warhammer 40K to make miniatures, there’s still a dedicated group of makers designing prints that are 1:1 scale for humans.
Nathan Williams has designed this bolt pistol in Fusion 360 and printed it to be perfectly scaled for cosplay or display. He used a Flashforge Creator Pro for the version you see in the gallery below, and you can obviously scale it up or down to suit your needs (and hand size).
The raw print was primed and then painted with an airbrush. The worn look was accomplished with black paint being rubbed into the red, and another round of sandpaper on the finished paint to scuff it up. It’s not 40K if everything isn’t grim and dirty.