A Florida sheriff's deputy is looking more like a cartoonish cop from "Reno 911!" on body cam footage of him mistaking an acorn hitting his patrol car for bullets ... prompting him to open fire on the vehicle.

The video, which was just released, shows Okaloosa County deputy Jesse Hernandez on the job, detaining a guy back in November for allegedly sending threatening texts. Hernandez and his partner cuffed the man and put him in the back of their patrol car.

All seemed to be going well, until Hernandez appeared to hear what he perceived as a shot being fired -- which catapulted him into a frantic drop and roll as he drew his weapon and started firing multiple shots at his car.

Hernandez seems to unload his entire clip ... and, remember, 22-year-old Marquis Jackson was helplessly sitting inside the patrol car, unarmed and completely restrained -- yet, Hernandez clearly thought Jackson had shot at him from inside the car.

Amid the panic, his partner -- who didn't hear anything like a gunshot -- seemed confused as to why he was freaking out, and also started shooting at their car.

Miraculously, both of them missed Jackson, but Hernandez's body cam video shows he completely overreacted ... which was the finding of the Sheriff's Office investigation. Okaloosa officials determined his use of deadly force wasn't reasonable, and his partner got a pass ... since she only fired based on his actions.

Over the course of the investigation ... Hernandez was made aware that what he thought he heard was literally just an acorn falling on his car's hood. Hernandez ended up resigning in the aftermath of all this ... and was not criminally charged.

From what we can tell, Jackson hasn't sued, yet, but has told local media outlets he was "damaged" from what he went through.

Once the dust settled, the Okaloosa County Sheriff's Office said this ... "We are very thankful Mr. Jackson wasn’t injured and we have no reason to think former Deputy Hernandez acted with any malice. We are limited in further response due to pending litigation. But let this be clear, we understand this situation was traumatic for Mr. Jackson and all involved and have incorporated this officer-involved shooting into our training to try to ensure nothing similar happens again."