For fun I like to play the game only with weapons that are accurate for the country of the team that you're playing on.

You can take this self-imposed challenge strictly and only use weapons that were actually used by the country you are playing as, so for example then you wouldn't be allowed to play Assault class as Japanese because they do not have any weapons in for that class.
Or you can do a more relaxed version of the challenge and say that allies/axis can share guns, so then British get to use American guns and Japanese get to use German guns.

As an added rule, you're always allowed to take your defeated opponent's guns.

Here I have an in-depth list of weapons and which countries used them to help you with this challenge:
Steam Guide - List of Weapons by Class and Country + Some historical facts

Good luck and have fun!