There are some aftermarket kits that give you all 4 positions - Safe, Semi, Burst, Full. I think Tapco sells them
as replacements for parts in a legally registered machine gun. According to federal law, MGs made and legally registered before 1986 may be sold to civilians. State laws vary.
Also, as a slight correction to an earlier post, "select fire" means capable of firing in either "semi-automatic" or "more-than-semi-automatic", where "more-than-semi-automatic" means either full-auto or burst (either way, the gun fires more than once per trigger pull). Either burst or full-auto is considered a machine gun by law. "select fire" is contrasted with either "semi-only", "burst-only", or "auto-only". There are some very simple machine guns that are full-auto-only and have no semi mode of operation.
As an aside, there are some crazy laws on the books. OH
defines a semi rifle with a mag holding more than 30 (32?) to be a machine gun, even though it can't do burst or full-auto. Some states allow "full-auto-only" but not "select-fire" (which gives you the choice between semi and full!)
Woody at work said:
From the horse's mouth... the M4 and M16 are available with EITHER full auto OR 3 shot burst (not both) and with EITHER fixed carry handle OR flat-top receiver/removable handle. Four distinctly different models of M4 and M16.