As I understand it, the critical difference is that they'd have to source or make (out of steel, and likely with heat treating) more parts, like the full auto s

As I understand it, the critical difference is that they'd have to source or make (out of steel, and likely with heat treating) more parts, like the full auto sear, and make an additional cut and hole in the lower receiver. Here are some of them: http://www.brownells.com/rifle-parts/triggers-parts/sear-parts/index.htm?avs%7CMake_3=M16 http://www.brownells.com/rifle-parts/triggers-parts/sear-par... (There are many, many not so big differences, especially as these have evolved over they years, and then there's the newer M4 carbine variant, which I'm told also has some important general reliability improvements.) Of course, buying these, especially in large quantities, would likely attract the wrong sorts of attention. One motivation they might have for making their own lower receivers from billets is to stay off the radar; normal AR-15 parts and ubiquitous, seeing as we started using it as a service rifle in 1962-5, and we civilian in the US own probably around 6 million.