How do you figure?
The Miller case determined it was perfectly appropriate for the government to regulate weapons that had no millitary purpouse. They claimed the second amendment only protected weapons with a military purpouse. The case applied that to probably the favored weapon in the trenches of WWI, the sawed-off shotgun.
Therefore, I would suggest, using the same logic, the .50 is perfectly suitable for government bans since it is undeniably used in military combat, it must have no military purpouse...
Not quite. It applied to a double barreled break action sawed off shotgun, a Stevens, IIRC. Not a 18" (M 1897) or 20" (M 12) barrel (most were longer) pump shotgun with bayonet lug and bayonet.
Furthermore if you read the decision closely, you'll see that that at most the court indicated that it was not within judicial notice that such a weapon (IE. Stevens break action short double barreled shotgun) "was any part of the ordinary military equipment" or that is use or possession could "contribute to the common defense" and that the "The cause will be remanded for further proceedings. Those further proceedings would have undoubted found many instances of the uses of double barreled short shotguns for military uses, in the Spanish American War for instance, perhaps even in the Banana Wars, then concluding in Central America. Unfortunately such proceedings were never held. The core of the conclusion/ruling of the court is below (less the "further proceedings ruling):
In the absence of any evidence tending to show that possession or use of a "shotgun having a barrel of less than eighteen inches in length" at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. Certainly it is not within judicial notice that this weapon is any part of the ordinary military equipment or that its use could contribute to the common defense.
Clearly implying that if such a weapon were shown to be "part of the ordinary military equipment" and/or that it's "use or possession could contribute to the common defense", it's keeping and bearing, by individuals, is protected by the second amendment. Kinda throws the machine gun portions of the NFA, the various state Assault Weapons Bans, the California .50 cal ban, and even handgun bans (Military M9 9 mm Beretta Pistol (concealable) and the H&K USP 45 caliber pistol) right into a three cornered hat, doesn't it?