The Miller case happened at a time when most people believed the NFA would be thrown out the first time it was actually challenged as unconstitutional.
Even most legal professionals that supported the restrictions did not believe it would hold up to Constitutional review. Records of the conversations and statements of educated prominent "antis" of the time show this.
Back into the 20s there had been a push by some to create NFA type restrictions, and for the same reason it never happened.
Then FDR happened. FDR could do anything, change anything, and made sure he got his way.
An example of this is most notable in his "Court Packing" threats and related proposed legislation the 'Judiciary Reorganization Bill of 1937' (Miller was heard in 1939). Where he would threaten to increase the number of Supreme Court Justices if they disagreed with him. Since the president would appoint a Justice to the new spots created it would insure FDR could always place a majority that would agree with him.
This was originally seen by FDR as necessary to pass his "New Deal" and similar controversial legislation which did not have enough support on the Supreme Court.
The Supreme Court eventually caved to his threats. By the end of his presidency he ended up with 8 out of 9 of the Supreme Court Justices being appointed by him anyways.
Now as it applies to the NFA and eventually Miller.
In 1932 the Bonus Army marched on Washington D.C. These were World War 1 veterans, unpaid well trained military vets suffering during the Great Depression who wanted payment for their service in World War 1.
Payment promised for a later date.
Eventually tanks and soldiers were sent out to disperse the crowd and destroy their makeshift camps.
The Bonus Army originally thought it was a parade to celebrate their military service being performed and cheered the arrival of the tanks and troops.
Tanks and bayonets made quick work of the protesters. As well as adamsite gas (keep in mind many soldiers had been subject to poison gas attacks in WW1 so the similarity probably brought back some memories).
The size of this force and their training scared Washington D.C. That they were defeated by being surprised with unexpected military force meant they could come back more dangerous in the future. Returning with the ability to resist such force.
Next time these men trained in Trench Warfare may come back with perfectly legal machineguns, submachineguns, and other modern weapons of war at the time.
As a result many worked from that point on to come up with a way to outlaw most aspects of the RKBA that could actually pose a threat or deterrent to the US government. FDR was campaigning for the presidency in 1932 when this happened and would be elected months later.
When Roosevelt took power in 1933 they eventually got to work on this issue. He directed his appointed Attorney General Homer Cummings to start work on legislation to outlaw most effective firearms.
Since even the Federal Government and the strongest of antis believed they had no authority to outlaw, they tried to play games and instead tax out of existence. Most pro gun and anti gun individuals still felt it would fail when challenged as an infringement, but in the meantime it was a way to ban.
They exploited infamous well armed criminals. But Bonnie and Clyde stole BARs from National Guard armories, John Dillinger stole Tommy Guns from Illinois police stations, and Ma Barker's sons acquired stolen Army weapons.
So the sources of most of the weapons of these infamous criminals were LEO and military sources that would be unchanged by the legislation.
It was all smoke and mirrors. An exploitative pretense that gave the illusion of being passed to counter those type of criminals.
The original NFA draft outlawed far more firearms types. Including all handguns. The scope was greatly reduced just before passage. Though that scope would again be increased in the 1968 GCA amendments that added new weapon types to the NFA.
Then we get to the first time it is challenged and appears before the Court in 1939. Most legal professionals still believed it would be defeated under Constitutional review. Even under FDR's Supreme Court.
However Miller dies before he can argue his case before the court. Nobody argues on his behalf. The Court takes the position that it cannot consider evidence never formally entered.
Since nobody was going to enter evidence showing short barreled shotguns were useful in a military capacity (they were widely used in WW1 so that evidence existed in great amount) consistent with the protections of the RKBA and militia type usage, it allowed the conclusion.
However the court essentially states that had the weapon been shown to be useful for such military/militia purposes its ownership would have been protected under the 2nd Amendment.
Clearly this means that machineguns and similar weapons that are clearly suitable for use in a militia are protected under that ruling.
It just required a new case where someone actually lived until they appeared in court. A machinegun case would actually be guaranteed to result in overturning the NFA under the Court's wording in Miller.
However then generations pass before the issue is ever addressed again. This is no accident, the SCOTUS picks and chooses which cases it hears. So if they do not want to revisit an issue they don't.
People having grown up never knowing what firearm freedom was now interpret the law and set precedent. So when Heller is addressed decades later they specifically look for a way to allow such restrictions to remain.
In the Heller decision the Court actually refers to the Miller case and removes any and all possible meaning in the Miller decision.
Scalia removes any legal power the words of the Court in the Miller case held, including that arms useful in a militia capacity would be protected.
As a result of Heller the Miller case no longer has the pro gun meaning it did. Heller overwrites all aspects of Miller, intentionally.
Under Heller there is new logic arguing both for and against gun control. Arms widely owned by the population are protected, yet "reasonable restrictions" are permitted. So rather than "shall not be infringed" it is 'Okay if Reasonable'. Clearly who defines what "reasonable" is defines the RKBA under Heller.
New technology can also never be widely owned before it is banned so it also most likely limits people to existing popular technology. So you can be denied ownership of a "phased plasma rifle in the 40 watt range."
