How to control power going from external battery to project from uno?

Hi I’m very new to Arduino electronics and stuff and I’m working on a project that detects when someone enters a room and activates a water gun to shoot. I’m powering the gun itself with a drill battery so my question is, how would I control when the power goes out from the battery to the gun with the uno? Again I’m very new to this so sorry if my wording is a bit basic or off. Any help would be appreciated. thanks! (Photo attached)

Hi! What´s the normal battery level of the water gun? What is the minimum voltage that should trigger the "low battery" condition?

I’m not sure but normally it runs off 4 AA batteries so I would think 6v

Nvm I’ve now learned that I need a relay!

diegstuff:
Nvm I’ve now learned that I need a relay!

You mean to trigger the gun?

It's better to use the relay module which can be directly connected to the Arduino without any transistor (the relay modules come with built in transistor). You shouldn't connect the relay directly to the Arduino because the relays need more current than the mcu in Arduino can provide, so it will just fry your Arduino