Monalisa Perez told police her partner Pedro Ruiz III wanted to film a video of her shooting a bullet into a book he was holding against his chest.
Mr Ruiz, 22, thought the book would stop the bullet, CBS affiliate WSPA reported.
Perez, a pregnant 20-year-old, said she fired the .50-calibre handgun from about a foot (30cm) away.
Mr Ruiz, who has a three-year-old daughter with Perez, died from a gunshot wound to the chest, authorities in Halstad, Minnesota, said.
The child was reportedly present when the incident happened at about 6.30pm on Monday.
Before the stunt, Perez wrote on Twitter: “Me and Pedro are probably going to shoot one of the most dangerous videos ever HIS idea not MINE.”
An air ambulance was called to the scene on US Highway 75 but Mr Ruiz was pronounced dead.
Me and Pedro are probably going to shoot one of the most dangerous videos ever😳😳 HIS idea not MINE🙈
— Monalisa Perez (@MonalisaPerez5) June 26, 2017
When police examined the scene they found two GoPro cameras had been set up – one on the back of a car and one on a ladder.
The victim’s aunt, Claudia Ruiz, told WDAY-TV the couple often carried out pranks and put them on the amateur video channel.
She told Valley News Live: “They were in love. They loved each other. It was just a prank gone wrong. It shouldn’t have happened like this.
“He had told me about an idea. I said, don’t do it, don’t do it. Why are you going to use a gun? Why? Because, we want more viewers.
“I don’t know why they thought the book was supposed to stop the bullet.”
She said the couple’s unborn child was a boy. “We’re going to name it Pedro, I want my name to carry on,” she added.
The couple’s YouTube channel, which they describe as “all about craziness”, has just under 2,000 subscribers. Video uploads show the pranks they have pulled on each other.
They include putting baby powder on a doughnut disguised as icing sugar, planting the world’s hottest chilli in each other’s sandwiches, as well as videos documenting their days out at the supermarket.
Their most watched video at a fun fair has had around 150,000 views.
Perez, who faces second degree manslaughter and reckless discharge of a firearm charges, was granted a public defence lawyer and released on $7,000 (£5,400) bail.
The charge carries a maximum penalty of 10 years in jail.