Regrettably, one cannot legally build a machine gun legally in the US. A damn shame, really. ATF regs for full-auto require it to be pre-1986. For artillery, destructive device laws are fairly bizarre - I think you're allowed the artillery itself, but each piece of ammo requires a DD tax stamp that will cost significantly more than the actual round.
Because any item that can be used to convert semi-auto to full-auto is considered subject to the aforementioned ATF regulations, for three years, shoestrings were considered machine guns. One can tie any length of string off to the bolt of, say, an SKS, and wrap it around the trigger in such a way that pulling on it results in fully automatic fire. The decision was reversed in 2007, and shoestrings were deemed machine guns only when used in conjunction with a semi-automatic rifle.
This is a legal, serialized shoestring machine gun from 1996: http://www.everydaynodaysoff.com/wp-content/uploads/2010/01/shoestring-machinegun.jpg