Andrew Hooper, known as Jack, pulled up on the drive of Cheryl Hooper's home, blocking in her Range Rover Evoque, on January 26 last year, before allegedly blasting her with a shotgun.
Hooper, described as "controlling" and "callous", is said by prosecutors to have shot his wife of 18 months in a fit of jealousy, after accusing her of having an affair.
The 46-year-old denies murdering Mrs Hooper, 51, in front of her teenage daughter as her 4x4 parked up on the driveway of her home in Newport, Shropshire.
Giving evidence at his trial on Wednesday, neighbour Wayne Tranter was preparing to go to sleep in an upstairs bedroom overlooking the scene when he heard two cars pull up outside, and the "aggressive voice" of a male.
He then jumped out of bed when he heard what he described as "two bangs", "like thuds".
Rushing to the window, Mr Tranter said: "I saw a man standing right in line with the door of the driver's side of the vehicle.
"The man was holding a gun and was pointing it towards the lady in the vehicle, in the position we all commonly know as the firing position."
He added: "I would say it was three-and-a-half feet from weapon to vehicle.
"My immediate response was to go and help."
As Mr Tranter pulled his boxer shorts on, he looked back out the window as the gun went off.
He said: "It was a split second.
"The firearm went off and the lady in the vehicle slumped to her left-hand side.
"I made a comment that doesn't need repeating, very quickly panicked and ran downstairs."
Mr Tranter said he heard the gun fired once.
Outside, he recalled seeing the silver Land Rover Defender in which the gunman had arrived "pulling off", and looked to see Mrs Hooper's daughter Georgia "come running towards me".
He said: "She was hysterical, she was screaming, she shouted 'my mom, my mom, he shot my mom' and 'that's my mom in there'."
Mr Tranter's partner Katie McMillan, who heard what she believed were two shots in "quick succession", comforted the victim's 15-year-old daughter and used the teenager's phone to call 999.
The recording was played to court, and in it Mr Tranter is heard telling the operator: "Her daughter is screaming in here, she's saying it's her stepdad."
Mr Tranter could also be heard being guided by the call handler, leaning into the Range Rover and shouting 'Cheryl, Cheryl, can you hear me?', but without a response.
He then told the 999 operator "she's not moving, there's nothing - she's shot in her right shoulder".
Miss McMillan said Mrs Hooper's daughter was "very distressed", by turns "screaming and crying", and then "very calm and apologetic".
Another neighbour Donna Campbell, who lived a few doors away, heard bangs, which she mistook for fireworks.
She heard "the sound of screaming", and Mrs Hooper's daughter shouting "oh my God, he's shot my mom.
"She's dead - what am I going to do?"
Hooper, of Guild Lane, Newport, Shropshire, denies murder and the trial continues.