Only way to convert any AR style rifle (complete rifle, upper and lower) to full auto, would require a
registered DIAS (
drop
in
auto
sear). Mechanically DAIS can't be converted to select fire (I'll put it this way, never seen one). Select fire would require a
registered receiver with all the parts (including the auto sear), either purchased as a package or separately. Restriction do apply.
If you do a search, I'm sure that you'd find a ton of detailed information that would better answer your question. Its been discussed here many times.
As for your Law Enforcement question. They can
own domestically produced machine guns made after the 86 ban, with department approval. Once they become PFC's once again, machine gun must be surrendered. Most all new machine gun sale today are for departments and not personnel. In the years we've been doing this, no department has ever signed off on a machine gun for LE personnel. And most of our sales of sub guns and rifles are in semi automatic form.
Gabriel