Chaz,
Not really working backwards. It's a matter of time and detail. The grips are actually very complex shapes. It would have taken me far longer to model them in the computer than it did to sculpt them by hand. Plus the "feel" of the grip was very important and being able to hold it while developing the shape was essential. Since the finished patterns need to have a gloss surface in order to yield clear castings, the progressive wet sanding by hand would have been required even if they had been completely formed with the CNC.
Likewise with the two side covers. The left side cover was completely modeled and milled on the CNC, needing only a little sanding to cleanup the cutting steps. But the right side cover, like the grips, is a very complex shape. So I modeled the basic form, cut that with the CNC and then hand sculpted the finer detailed curves. Again, it took far less time to sculpt by hand then to model in the computer.
As to just CNCing or 3D printing the whole blaster in two halves, the level of detail required and the desired castings make this impractical. This is going to be a pretty high end kit when done, and require some work to put together. Not just one of those $40 one piece castings you find on Ebay! Separate parts are also advantageous for painting and finishing, allowing different parts to be painted different colors or metal finishes, if desired and then assembled.
Several of the parts like the receiver, clip housing, bolt lever and thumb slides are being 3D printed. I should have the first batch of those parts back by the end of the week.