In terms of the most destructive thing you can own.
I believe in most cases the limit is in the definition of "arms" which varies but is limited in general to something a single person can physically carry.
Nukes are specifically limited and regulated to the Department of Energy.
Warplanes, tanks, ships, etc are not considered arms AFAIK. Similarly, you couldn't own something like a mortar setup that takes 2 people to operate even though you could likely get a carry weapon that has just as much firepower.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)