Possibly several different folks, maybe some for each side. A military historian and/or officer (who could easily be the same person) would be a good choice. The testimony would be to answer the question "had such a weapon been used in militia/military service or could it be" The question to "has it" is clearly yes, the answer to "could it" might depend on the person. If the officer was from the calvary, still using horses in 1939, and even later in some cases, including the US Army after WW-II in patrolling the inter-german and other borders in Europe, the answer quite likely would be yes. Of course the real answer was to come in just a few years, as related by the US first circuit court in Cases
the rule of the Miller case, if intended to be comprehensive and complete would seem to be already outdated, in spite of the fact that it was formulated only three and a half years ago, because of the well known fact that in the so called "Commando Units" some sort of military use seems to have been found for almost any modern lethal weapon. In view of this, if the rule of the Miller case is general and complete, the result would follow that, under present day conditions, the federal government would be empowered only to regulate the possession or use of weapons such as a flintlock musket or a matchlock harquebus. But to hold that the Second Amendment limits the federal government to regulations concerning only weapons which can be classed as antiques or curiosities, -almost any other might bear some reasonable relationship to the preservation or efficiency of a well regulated militia unit of the present day,-is in effect to hold that the limitation of the Second Amendment is absolute. Another objection to the rule of the Miller case as a full and general statement is that according to it Congress would be prevented by the Second Amendment from regulating the possession or use by private persons not present or prospective members of any military unit, of distinctly military arms, such as machine guns, trench mortars, anti-tank or anti-aircraft guns, even though under the circumstances surrounding such possession or use it would be inconceivable that a private person could have any legitimate reason for having such a weapon. It seems to us unlikely that the framers of the Amendment intended any such result.
But of course, that is exactly the result intended by the Framers of the Second Amendment. But the first Circut substituted its policy preferences in place of the judgement of the Supreme Court and the written words in the Constitution itself, not to mention the other writings of the Founding Fathers.
That's not what the Miller court asked! I am getting really PO'd with people rephrasing the argument, yourself included.
The court asked if the weapon bore some reasonable relationship to the preservation or efficiency of a well regulated militia -- NOT if it had "some sort of military use".
At the time of this Supreme Court inquiry, WWII was years away. Why are you even bringing it up? Cases was years away. Why bring it up?
At the time of Miller, WWI was concluded. The military ordered and used tens of thousands of "trench guns" for the military. They were 20".
Im my opinion, Mr. Miller's sawed-off shotgun would NOT have qualifird.
Lastly, let's say this military historian/officer testified and said, for sake of argument, that a shotgun less than 18" was not suitable. You realize, therefore, that Congress could then prohibit such a weapon (no second amendment protection!) meaning that no state militia in the United States would be able to use it?
One military historian/officer making that decision for militias acrosss the United States? I don't think so.