Karl, I wanted to ask a question about manufacturing methods for the
Ar-15 receiver.

Look at this:

http://www.cncguns.com/projects/ar15lower.html

He had to reclamp his piece numerous times to make it on a CNC mill. I
wonder, how much of that is due to lack of the 4th axis.

My feeling on this is that with a suitable 4th axis, this receiver could be
done in 2-3 reclampings, or perhaps even one. With a 4th axis, clamp
once, and then turn various sides towards the spindle to be machined.

Am I making any sense?

i