Luke Adam Nickless is on trial at the Christchurch District Court, accused of strangling Renaye Strachan and shooting her in the leg last September at his Loburn property.
Nickless acknowledged using the gun was "a stupid idea", but said he just wanted his ex-partner to calm down and talk to him.
"I was just meaning for the sound of it to scare her," said Nickless.
He had asked his partner to bring her two boys, one who is his son, to his property earlier that day to ride mini motorcycles.
Nickless said a struggle had broken out between the pair when he removed Strachan's cell phone from her pocket and tried to look through it.
He said Strachan grabbed his bicep pinching it with force while kicking and hitting with him with her other hand.
He said he had pushed her back with an open palm but denied choking her.
When Strachan went out to her car to leave with the children, Nickless said he took his shotgun from a wardrobe.
"I just wanted her to engage with me and sit down and talk to me," Nickless said.
"I never aimed that shotgun at Renaye."
He said he went to pump the shotgun so it would "make the sound", and it went off in his hands, shooting his ex-partner in the leg.
Before he gave his evidence, Nickless pleaded guilty to the charge of unlawful possession of the cut-down pump-action shotgun he used in the shooting.
Nickless told the court he was sure he called emergency services after the shooting.
However, Detective Matthew Reynolds, said there was no record of Nickless calling 111.
Phone records show a neighbouring property had called the ambulance after the shooting.
The trial continues.