He then raised the gun and aimed it at the victim's head and pulled the trigger, Christchurch District Court heard today.
As the gun went off, the victim flinched and was shot in the neck.
It caused him to feel instantly dizzy and stumble backwards, falling to the ground.
Grimwood and his mate drove off.
Police launched a manhunt and issued a photo of Grimwood, naming him, and warning the public that he "should not be approached".
He was caught up with days later and charged.
Today he pleaded guilty to wounding with intent to cause grievous bodily harm, along with a raft of firearms charges.
Judge David Saunders remanded him in custody to be sentenced on April 22.
When police caught up with Grimwood, he admitted the shooting.
He told officers that he fired the gun at the victim because he'd taken a car from him and he thought he was a "gangsta".
The victim suffered a gunshot wound to the right side of his neck.
The bullet is lodged in the top of his rib and will require surgery to remove, the court heard.
His condition is described by medical professionals as serious but stable.