David Manilla, 51, of Worcester, entered the plea to possession of firearms by a convicted felon.
The U.S. Attorney's Office said Manilla was found to be in possession of multiple firearms and ammunition during the investigation into the killing of Barry Groh.
Manilla, who was hunting illegally, mistook Groh for a deer and fired the fatal shot.
He was prohibited from possessing any weapons because of his conviction for aggravated assault for the 1985 beating of another man, the U.S. Attorney's Office said.
Manilla is scheduled to be sentenced in June. He faces up to 10 years in prison and a fine of up to $250,000.