The limitations for mailing a firearm is on concealable Firearms. Long guns are not considered "concealable". If you mailed your machinegun with a 16" barrel and a stock, that would not be considered concealable. Registered Mail is by far the most cost-effective way to ship a machinegun, as commercial carriers must ship overnight.
Scott
I mentioned the macmachineguns.com guys to someone on reddit the other day and they called and were told there's a 6 month wait on any work.