Police discovered the clip online after they arrested 19-year-old Michael Barnes - whom is dating the little girl's mother - and searched his phone.
At one point in the footage, the child, who has not been named, can be seen pointing the gun into her mouth.
As she disturbingly shouts out the word 'pow' whilst playing with the gun, it soon becomes clear that the baby is being supervised by both Barnes and her 22-year-old mother Toni Wilson.
In fact not only supervised, but encouraged; a voice can be heard saying "shoot that gun, fat baby" from off-screen.
Watch the footage for yourself below:
Both Wilson and Barnes have now been charged with child neglect.
Sergeant Jason Cullum, from the local police force in Indiana, USA, described the video as “mind-numbing”.
He said: “It just makes you wonder how many other children may be living in this type of situation and what the outcome is going to be.”