A Nebraska pup apparently took "shotgun rider" way too literally ... because cops say the dog accidentally fired a real shotgun from inside a parked truck.

The bizarre incident went down Saturday in Scottsbluff ... where police responded to reports of a shooting near a convenience store and discovered a truck with damage from a shotgun blast ripped through the passenger-side door.

Law enforcement officials tell TMZ the alleged canine culprit -- who is reportedly a black lab -- was left inside the parked truck while the owner stepped away.

According to investigators, the truck's owner stepped inside the store to buy some items ... meanwhile, the dog sitting in the back seat allegedly moved from one side of the truck to the other ... and somehow triggered a loaded shotgun.

The gun fired through the door ... and one pellet struck a woman who was stopped nearby at a traffic light with her arm hanging out the window.

Thankfully, police say the woman's injury to her upper right arm was not life-threatening -- and a family member drove her to a hospital for treatment.

Cops are still investigating the freak accident ... and also reminded folks it’s illegal in Nebraska to travel with a loaded shotgun inside a vehicle.

Safe to say this pup’s probably not getting promoted from back seat to shotgun anytime soon.