It was illegal for Michael Allen to supply a gun to Roderic Dantzler because Dantzler was an ex-convict who wasn't supposed to have weapons.
Allen is a 35-year-old Grand Rapids resident and faces up to 10 years in federal prison when he returns to court on Nov. 18. He pleaded guilty Wednesday.
Dantzler killed seven people during a shooting spree, including his estranged wife and their 12-year-old daughter. He also killed himself.
Federal prosecutors said ex-convict Joseph Krul pleaded guilty in May to possessing the same gun before Dantzler got it.