The best way to do this is to find a friend, go to Server List -> Community -> Filters, type in the name of the Server: “Zavod” and choose ZavodTest (or any similar PvP server with close respawns). You always spawn 10 meters away from everyone else.
You should have few backpacks with a weapon right near your respawn on the ground. Take it, reload it, and go to the main road with a friend.
Then you have to get as far as you can from each other, lay down, press LS to hold breath and shoot.
Me and my friend did it with AKSU, the most common AR/SMG in the game, without any scopes etc.
Take your time, hold your breath, shoot 1 bullet at a time and you’ll get it in no time.
Just make sure that you only see few pixels of your friend as 300 meters is really a lot hehe.