It takes me 3 hours just to solder one together, then there's the accessories and wiring harnesses and testing to do. I easily lose an entire day assembling, testing and configuring one complete unit, and I don't have much spare time between my job and personal life.
Honestly, 3 hours sounds about right. I can attest to these things taking time, and time to assemble them right. Wiring harnesses alone can eat up a good chunk on their own just on the crimping.
You mentioned previously getting an oven and a solder stencil made; depending on how quick you're soldering your surface-mount components this may or may not shave off some time to assemble. When I was doing some PCB assembly of one of my projects (USBemani, which was a long while ago) I ended up using a stencil, solder paste, and an electric skillet to do my reflow work to some pretty great results. I picked up a skillet on sale for about $20 USD, and that thing saved me a lot of time in the 200-or-so PCBs I assembled and sold.
Space becomes an issue with larger boards (I could fit about 12 in the skillet, I imagine for the supergun you could fit 2), but if your surface-mount stuff is taking your time it's well worth trying to find one cheap. I probably would've gone for the oven route, except converting those things can be a weekend project on their own.