The Fourth Circuit will find itself at odds with the SCOTUS Miller decision, which held that civilian-owned firearms “that are usual to military use” are protected under the Second Amendment. Recall that the Miller decision held that a sawed-off shotgun was not usual to military use, and thus was not a weapon protected under the Second Amendment.
When your quest is a predetermined decision, and your word is likely final, then any rationale will do.
The Supreme Court did NOT decide that the shotgun was not useful. Such a determination would require fact-finding which is more properly a function of the lower court. That is why they reversed and REMANDED the case.