Phyrbird, AFAIK printers that can do metal parts (sintering) are still a bit pricy. Of course what is pricy to me might be nothing to others. I know they use them in aeronautical industry but I do not know if they can produce load-bearing pieces.
Good site for that is http://www.practicalmachinist.com
With that said, a lousy little printer like mine (that can only print plastic parts) probably could be used to produce the original piece to create investment cast mould.
CAD programs are many. The gold standard is the French one, solidworks. Autodesk has a free one a lot of people like because of its simple interface and ability to do organic surfaces. But it does phone home if that matters to you. On the free side, there is Freecad, openscad (feels like a programming language instead of a normal cad), and even blender (more of a 3D program for animations than for CAD). I did the part in the picture using openscad because I can be exact and can do a bit at work without anybody being the wiser.
After the object is done it needs to be converted into what is called g-code, which is the language most 3D printers and CNC mills understand. There are programs to do that, but getting it right for a given CNC machine requires understanding said machine sometimes.
FYI, my goal is to get the model properly done and then get it to a CNC mill and feed it a block of aluminum to see if everything fits. The real thing should be closer to what they use in AR so-called barrel extensions and AK/CETME/STG57?