The Samurai Edge Handgun is one of the best weapons to take out zombies, but it's even better to get the "Infinite Ammo Samurai Edge Handgun" that, as the name implies, never run out of ammo!
Getting the Infinite Ammo Samurai Edge Handgun can be very difficult, so be prepared for that before getting too invested with it. Odds are that most players won't be able to get it unless they invest a lot of time getting good at the game. This is because you need to finish the game with an S rank to get it.
Getting an S rank for your entire playtrough is actually quite difficult, and if you play on Standard difficulty, it means that you must finish the entire game from start of finish in under four hours. However, this only applies for your first run! In your second, and all following runs, you will need to beat the entire game in under 3 hours in Standard mode.
If you manage to do this, you get the famous S rank, and you will be awarded with the Infinite Ammo Samurai Edge Handgun.
You will be able to tell if you did it, because you will be greeted with a message in the main menu after you complete it. It will tell you that you unlocked the Infinite Ammo Samurai Edge Handgun, and at that point all your saves will have this gun with the unlimited ammo in an item box.