In Cases v. United States, 1942, a three-judge panel on the First Circuit ruled it was unlikely Miller meant military arms were protected by the Second Amendment: From Cases:
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.
The judges did not want military arms protected, so they ruled they were not protected.
Thanks for that info.