Thank you. I remember the upper and lower receiver groups of the M-16. I learned about them in basic training 40 some years ago.
“The DD files made it very simple for an individual to buy receivers that are 80% done, and complete the construction using a small consumer CNC machine. “
While I think this stinks, building an AR through the use of an 80% receiver is easy. You can, if you are careful, complete a receiver with a drillpress, but a small benchtop mill ($700 at a half a dozen places) gives you a functional part that looks just like the commercially machined ones you have to buy through an FFL. Some places that sell 80% lowers also sell “machining jigs” to help you locate all the required features. Everyone should have “his or her own AR!”
And now you can do the very same thing for 1911’s and some of the Sigs.