The gun was found with 30-year-old Eric Ramsey after he was killed early Thursday by a Crawford County sheriff's deputy.
The abduction happened Wednesday on the Mount Pleasant campus.
Michigan State Police Lt. Derrick Carroll said the gun was made to look like a .45-caliber weapon. He said there was no way to know it was a BB gun.
Ramsey drove the woman in her own vehicle to an off-campus home and raped her. Authorities said Ramsey put her back in the vehicle and told her he would kill her, but she escaped and ran to a home.