Step 1: The Concept
The original concept was to make use of the growing technology of 3d printing; to model the complete weapon in 3d and print out the parts that linked several military surplus parts that looked cool for the project.
It ended up with me doing serious metal working in my apartment, with inadequate tools. The biggest tool I had was a budget, bench top drill press from Harbor Freight. Most of the serious cutting was done with a hacksaw!
I also made some serious errors, such as being misled by my experiences with welding steel to think I could braze multiple brass parts together in the same way. In the end, the thing was as much epoxy and other glues as it was metal, and in hindsight we really could have printed more of it, done a lot less metal work, and saved ourselves much time and money.
Step 2: Importing the Parts
The other parts were done by hand...calipers in hand, frequent measurements, lots of playing with scale tools and grid settings. Early on I set the project in metric...and then it turned out almost all of the important parts lined up better in Imperial units. Not only that, there is STILL a bug in Carrara Studio wherein if you set the workspace to metric it multiplies all numeric entry by 2.54 (the number of centimeters in an inch, natch).