This is the updated version of my Carnifex Hand cannon. I've printed off a version of this and I'm documenting my build on several different pages.
That's where you can see most of the work I've put into this, and where you can see how they've worked when I printed them. I cut the main body into smaller parts to fit on smaller print beds, and in the hopes it will reduce warping for those of you who print it.
StarJeff is designing a display stand for this, available here:
REUPLOADED FILE: http://www.thingiverse.com/thing:1850891
Update 8-4-2015: I made a second one of these, and gave it to Rana McAnear (the face model for Samara/Morinth). You can see some shots of it on her various social media profiles, and it will hopefully be making the convention rounds with her.
I added a few holes between the parts to stick short pieces of 1.75mm filament in, to help you align the parts as you glue them together. For PLA, I used some plasti-weld cement, which is pretty gnarly fume-wise, and awful for your health. Gets the job done, though, and the parts are physically melted together at the seams. If you're using ABS, then you can use an acetone-based model cement to accomplish the same thing.
For a more detailed version of how I've handled the post-print process with the two I've made so far, the link to my build thread on therpf forums has much more information.
I designed this in Solidworks. I started with a rough outline from a blueprint and reference pictures from in-game, then traced it in a sketch, and extruded it to the width I wanted. That gave me the rough outline of it, and the rest of it was a matter of adding more layers of extruded solids or extruded cuts to add details. The curved section was considerably more complex than that, but the main body of it was mostly just 2D shapes stretched to size with details cut in.
The circular holes in the top and bottom of the main body hold 1/4" magnets, as well as one near the top by the battery, to later hold on a scope attachment.
The upper barrel is set to take a press-in threaded insert (http://www.grainger.com/product/Ultrasonic-Insert-1GRL3 is what I used). I'll be modelling and making different attachments that you can screw in using a 3/8-16 bolt. As seen with the fake silencer I've posted in the build thread.