It is not easy, unless you are a machinist and have the tools and parts needed to do it. The receivers on civilian AR-15 lower receivers would have to be milled to accept the sear. It is my understanding that the fire control group parts and the sear needed for the conversion to full auto are restricted and not available to your average person. I believe the full auto bolt carrier is also restricted but I am not sure about that. I ran a small arms repair shop in the Army for a few years and have looked at what it would take. It is beyond my skill level to do.
Just a question about select fire. Don’t outfits like the one on Sons of Guns do conversions all the time. They deal in some very heavy firepower. Didn’t they make a quad mount for MG 42s?