Battlefield 1's teensy Easter egg isn't so much deadly as it is hilarious.
The ultra-tiny 2mm Kolibri, also known as the "world's smallest commercially available centerfire cartridge" handgun, will be a useable gun in Battlefield 1. As you can see in the GIF, the Kolibri, which is named after a hummingbird, isn't exactly lethal so there's no actual tactical reason to use it, but your opponents might actually die of laughter upon seeing it.
The Kolibri is actually a real gun that was made the same year World War I started, so it actually fits chronologically. To me, the funniest thing about the gun is its cocking animation...and the fact that you'll be holding it with your pinky up to ensure proper etiquette.