U.S. 2nd Amendment

[David]

The law was written in the late 1700s so those are the only guns you should be allowed

if they're allowed to use (for whatever weird reason) guns made over 200 years later than what the people who wrote that rule had in mind

 
There's no weird reason, the idea is that civilians have access to the same small arms as the military which obviously changes over time. In the unlikely event of a tyrannical government you should be on an even playing field when it comes to person-person combat