1. Holster a primary or two-handed weapon (M4A1, Mosin, SKS, Unsawed Shotgun, etc.) on your character's back and assign it to a number hotkey (ie. '1').

2. Take a second primary or two handed weapon into your character's hands and assign it to a number hotkey (ie. '2').

3. Press the assigned number hotkey of the weapon holstered on your character's back and watch as the weapon in your hands drops on the ground (or is lost in some stairs or tree branches or swallowed by a server restart) as the holstered weapon is drawn.

This could be remedied by having the primary weapon in hands go directly to the holstered position if the player switches to a holstered primary weapon using the number hotkeys. In the inventory screen