Laws about gun ownership differ around the world.
United States of AmericaEdit
In America, you can legally own a variety of guns, including sniper rifles and machine guns. Different states have different laws, and some local ordinances are in place at various locations.
For as low as 400 dollars you can get a semiautomatic gun with a thirty round clip of ammunition.
You can legally own a semi-automatic civilian legal M50 with a magazine capable of holding one hundred rounds of ammunition and capable of shooting from 1 to 2 miles distance.
You can legally own various types of explosives in America. Some laws involving how they are stored are in place.
Wikipedia:Tannerite is a cheap binary explosive sold in hunting stores in 49 states of America, with only the state of Maryland outlawing it. It is mostly used for target practice, do to the coolness factor of shooting something with a rifle and having it explode.
A bulldozer can be turned into a bullet proof vehicle of destruction. Take the example of Marvin Heemeyer who used steel and concrete to armor up a bulldozer, with bulletproof plastic over security cameras he used to see around with, and then went and demolished the town hall and other buildings before getting caught in a basement and unable to drive off. For more information see Wikipedia:Marvin_Heemeyer.
Should the world come to an end, people may make such vehicles, as bulldozers won't have to worry about flat tires from broken glass or metal on the roads or other problems, and they can easily have a gunhole to shoot a machine gun or high powered rifle out of. The only way to stop them from tearing into your home or community, would be to catapult explosives at them. A moat would slow them down, but they could just keep bulldozing stuff into it in order to cross. If they didn't have a supply of oxygen to breath, then even if they filtered the air they couldn't be smoked out, you could still stop them with fire weaponry, flamethrowers, or catapult or throw alcohol or gasoline at them, and keep burning the air around them away faster than any could get in for them to breath. You could also hurl something to stick up their air vents. The vehicle will most likely have an exhaust pipe you could attempt to stop up.
Easily stolen tanksEdit
Surprisingly, some tanks don't require a key to turn them on, anyone able to use a crowbar to get a hatch open to gain access, and just pushing a button to start them up. This is what happened in the case of Shawn Nelson on May 18, 1995, when he stole an M60A3 Patton tank from a United States National Guard Armory. He then went on a rampage with it, before getting caught on the interstate concrete media divide, and a police officer got on top and shot him through the open hatch. For more information see Wikipedia:Shawn Nelson.
What military equipment can be stolen from National Guard units or elsewhere by anyone, or by corrupt members of the military for their own agendas, remains unknown. Just one more thing to worry about and a good reason to remain hidden if doomsday comes and the world goes crazy.