Mark Manes pleaded guilty to supplying a weapon to a minor and possession of a sawed-off shotgun. The ammunition purchase, which was not previously disclosed, was legal, prosecutor Steve Jensen said.
Manes told authorities gunman Eric Harris asked him to purchase the ammunition.
When he delivered the two boxes, "Manes asked if he (Harris) was going to go shooting that night and he said no, the next day," Jensen said.
Harris, 18, and Dylan Klebold, 17, killed 12 fellow students, a teacher and then themselves April 20 at Columbine High School. It was the worst school shooting in U.S. history.
At the time the two bought the gun from Manes in January, Harris also was 17 and thus underage.
During a brief court appearance, Manes looked mostly at the floor, answering District Judge Henry Nieto's questions with "yes" or "no." His parents sat in the front row of the spectator section. Manes faces from one to 18 years in prison when he is sentenced Oct. 14.
Attorney Robert Ransome, who represents Manes, said the family wants to take responsibility for Manes' role in the crime.
Manes is one of two people who were accused of helping Harris and Klebold acquire the TEC-DC 9 semiautomatic pistol - one of four weapons used in the rampage.
A former Columbine student, Manes purchased the weapon at a gun show last fall. Philip Duran, a pizza shop employee who worked with Harris and Klebold, is accused of introducing them to Manes when Duran learned they were looking for weapons.
Duran, 22, is scheduled to appear in court next week. He is charged with providing a gun to a minor, which carries a maximum sentence of six years in prison and a $500,000 fine.
Harris and Klebold also used two shotguns and a rifle legally purchased by Klebold's friend Robyn Anderson. Under Colorado law, an 18-year-old without a felony record can furnish minors with rifles and shotguns. Investigators have characterized Anderson as a witness, not a suspect.