The 31-year-old made two attempts to obtain the Russian-made fully-automatic weapon from an arms trader in Luxembourg in 2014, state prosecutors in Paderborn, North Rhine-Westphalia, said on Thursday.
Rather than present a German passport, the man presented a 'nationality document' from a supposed 'Free State of Prussia'.
After the dealer alerted police back in Germany, they raided a number of houses and apartments in the Höxter district near Paderborn, North Rhine-Westphaliia.
But all they found was a small amount of ammunition.
The suspect himself said that he wanted to acquire the weapon so as to found a police force for the 'Free State of Prussia'.
But he will now be prosecuted for infringing on the law controlling weapons of war.
Prussia long-since defunct
Prussia was a historical reality – first as the kingdom ruled by the Hohenzollerns, who became Emperors of the united German Empire in 1871, and then as a 'Free State' after the Kaiser's abdication at the end of the First World War.
Prussia, whose capital was Berlin, extended from central Germany all the way to modern-day Lithuania, as well as controlling different parts of the west German Rhineland throughout its history.
The Prussian state flag from the German Empire period. Image: David Liuzzo/Wikimedia Commons
But it has not existed as a political unit since 1947, when it was abolished by the Allied Control Council that took over management of what had until then been Hitler's Third Reich.
The Allies believed that Prussian culture had included a particularly dangerous strain of militarism that had inspired the Nazis in their ambitions to conquer all of Europe.
Conspiracy theorists dream of Reich
While some more sympathetic historians in Germany and abroad have argued in recent years that Prussian imagery and culture were twisted and abused by the Nazis into something very different from its original form, others within Germany have incorporated it into their conspiracy theories.
It's a favourite hobby-horse of the so-called "Reichsbürger" (Reich Citizens) movement.
Reichsbürger believe that because there was no peace treaty, the Second World War never really ended, that the Federal Republic of Germany is thus illegal – and that the German Empire therefore still exists and should rightly regain the borders it had in 1937.
They often refuse to follow the laws or obey the authorities of the Federal Republic and have been known to refuse to pay taxes and create their own identity and other papers.
Some Reichsbürger adherents have links to violent far-right groups and are under surveillance by the security services.