Youtuber Danger Doc has made one of the best looking custom replicas around, and shows in a video all the build process.
The gun is made of 3D printed parts, electronics, a premade gearbox, and some other small components. Some parts of the build require a lathe and a very specific plastic curing toy oven, but with enough ingenuity those could be done differently or bypassed in other designs.
The gun shows that a great deal of planning was done before building, as the body is very compact and everything fits together nicely. While the video is straightforward, the author states that 3 years went into it.
This pulse gun boasts a full auto system, with a spring reloading system that uses two neat tricks to prevent BBs from getting clogged (a small flexible lever and a gear with bumps). To add realism, the gun has lights and movement of the lateral barrel.
Oh, and it can shoot tracer rounds by lightning BBs as they are shot with 4 LEDs. The gun is wielded by Tracer, after all.