Anything an army is expected to field in a modern war.
That rules out terror weapons (WMD's) because they are a logistical nightmare.
Nukes are not just expensive, delivery systems are often toxic, high maintenance affairs.
The question itself is a problem, since it plays into the hands of gun grabbers who think just because WMD's may not be reasonable for private citizens to own, neither should handguns be considered reasonable, or semi-automatic weapons.
I think you can argue the right to bear arms refers to small arms up to battlefield munitions. Looking historically you have to include artillery and naval weapons as a possible "max limit" given how merchant ships could be armed.
WMD's are not generally deployed as battlefield weapons. (exception - some chemical weapons are in fact deployed all the time and legal. Tear gas and other agents desires to incapacitate are clear examples)
my simplest answer may be this: a citizen should be able to own enough firepower to remain a citizen and not a subject, serf or slave. That means the "max" firepower is not fixed, but fluid and evolves with the times.