They are "arms" within the meaning of the term in the second amendment. (It was not unusual for private individuals or private associations to own cannon or cannon armed ships at the time the second amendment was ratified, and even for several decades afterwards). So yes, we should be able to buy outdated Army tanks, or new ones from the manufactuer, if we've got the scratch.. which very few would.
I agree, I just wouldn't encourage it for most.
I worry about most of the idiots out there with just regular cars. ;-)