A federal permit is required to purchase a fully automatic weapon.
If your state or other local jurisdiction doesn't outlaw them. The 'permit' is a $200 transfer tax, an extensive federal background check including fingerprints and requires what essentially boils down to the permission of the local chief police officer. Once obtained you need written permission well in advance from a federal government agency in order to transport it over the state line... so you'd better choose your shooting range very carefully if you live near the state border. The same rules apply to rifles under 16" barrel length, shotguns under 18" barrel length, sound suppressors and 'destructive devices'. Note that a lot of these categories can be held quite readily on a UK firearms certificate.
Fully automatic weapons (real military assault rifles) can not be produced for civilian market anymore so what is out there is very expensive not to mention the federal transfer tax. M16s are going for $15,000+.
The registry for machineguns was closed under legally dubious circumstances in September 1986. A surplus Sten Mk3 receiver couldn't be sold for $200 in the 80s. Nowadays you're talking $5000... with two decades of wear and tear. The crucial/keystone part for converting HK rifles to full auto apparently are one of the most expensive pieces of metal, in terms of dollars-per-gram.
Most people buy the semiauto AR15 versions that are still produced and plan on converting them to full auto to fight off the zombies when they attack. Only one easy to make part is needed.
If they are 'readily convertible' the BATF will rule them a machinegun irrespective of the current configuration of the gun. The BATF is quite 'creative' when they deice on what is and isn't 'readily convertable'. One example that comes to mind require 24 man-hours of work by an expert gunsmith in a well-equipped gunsmithing-oriented machine shop and a number of part changes. (They also once ruled a shoestring a machinegun...) Conversion of a commercial semi-automatic rifle or pistol to full-automatic is actually quite difficult. The AR15 for instance (what the M16 and M4 are based on) requires the changing of several internal parts as well as the modification of the receiver. There are ways to get some guns to go full auto more easily but there is a significant chance of the gun blowing up in your hands and taking large chunks of you with it.
I also remember that if you had a Pistol Parabellum P'08 or Mauser C96 you needed to remove the lug that the shoulder stock fitted to, to avoid the need for a Federal permit and payment of the transfer tax. Does it still apply?
Jim
Generally yes, they come under the rules for 'short barreled rifle', for which the whole National Firearms Act process is required (transfer tax, paperwork, chief police officer permission, paperwork, fingerprints, more paperwork etc.). However a number of historical firearms have been exempted by the BATF from this paperwork under 'curio and relic' status. Examples include the 'artillery' model Luger (P08 with longer barrel and detachable shoulder stock/holster), certain models of C96 'broomhandle' Mauser with a similar setup, and models of P35 Browning Hi Power / Grand Puissance with a similar setup (some apparently with ladder rearsights graduated up to 1200 yards!). They are quite specific with regards to manufacturer and serial number ranges.
Quite a tangent!