1. Find an online match or create one yourself

1.
Select Tachanka and Equip his shotgun (or have a friend use a shotgun or any weapon/gadget that can destroy trap doors).

2. Find any trap door and destroy it making a hole through the floor.

Hold Tachankas LMG without deploying it.
Then simply walk into the hole and deploy it as soon you hit the ground.
You should be spinning around when you have deployed the LMG(It is possible to do it without the spinning animation).
Simply mount it and aim to your body and BOOM.
Congratulations.

If it didn't work the first time just simply pick up the LMG again and jump down the trap door like before.