The woman was being subjected to an "horrific assault" by Andrew Cullen and when Sergeant Tony Webb gave chase, Cullen pulled a handgun on the officer while trying to enter his front door.
Three armed response officers, who had been patrolling nearby, managed to detain gunman seconds later.
They had been alerted to the commotion by a member of the public and rushed to the scene arresting Cullen at gunpoint.
The dramatic encounter was captured on officers' body-worn video and subsequent checks on the gun revealed it to be "a realistic-looking imitation weapon" – but in a dimly-lit Birmingham street near midnight Sgt Webb said he believed the threat was real and was about to be shot.
Yesterday 42-year-old Cullen, of Mason Road, Erdington, was jailed for three years and three months after admitting possessing an imitation gun with intent to cause fear of violence plus assaulting the woman.
Sgt Webb, who works with West Midlands Police's Organised Crime and Gangs Unit in Birmingham, said: “It was a very scary situation - I thought the man was pointing a viable firearm at me.
“I’ve recovered and handled numerous firearms during my police career and there was nothing in the appearance of the gun that gave me any impression it was anything other than genuine.
“Thankfully the ARV (Armed Response Vehicle) was close by and the officers arrived within a few seconds.
“Looking back at the footage it demonstrates the amazing decision making and restraint shown by the three firearms officers.
"They needed to make split-second decisions when faced with an armed threat and dealt with it superbly.
“Cullen is clearly a man who is willing to use serious violence; I’m hugely grateful for the speedy arrival of my armed colleagues.”
Chief Inspector Jason Nunn from the force's Firearms Unit, said: “This video demonstrates the split second decisions officers are faced with, in fast moving and dynamic situations, every day when encountering armed or potentially armed subjects.
“It’s a testament to their professionalism and the high level of training they receive that they were able to bring this incident to a safe conclusion.”
The incident happened just before midnight on April 13 when Sgt Webb and a colleague responded to reports of a disturbance in Erdington.
As they entered Orphanage Road they saw a man grabbing a woman round the neck before throwing her to the floor and kicking her in the ribs.
Sgt Webb chased Cullen and caught up with him in Mason Road where he produced the handgun while trying to enter his front door.
Investigating officer, Detective Constable Ian Phillips, said: “The woman was subjected to a horrific assault: she was thrown quite some distance by Cullen, banging her head, followed by two strong kicks. She was lucky not to be seriously hurt.
“It was a vicious, cowardly attack and it’s right Cullen has been handed a jail term.”