Judge Richard Schoenstedt said prosecutors failed to convince him that Peterson shouldn't be allowed to own the AR-15 rifle that was taken from him three years ago.
"Under different circumstances, the officers of the Bolingbrook Police Department would have been able to possess and conceal carry this very weapon," Schoenstedt wrote.
"It was absolutely the correct decision to make," said Peterson's attorney, Joel Brodsky, according to the Chicago Sun-Times. He said the law allows police officers to "carry almost any type of weapon."
The charges were dropped a little more than a year ago by the same judge after prosecutors refused to surrender requested documents. Peterson's attorneys had argued the gun charges amounted to vindictive and selective prosecution.
That July 2009 decision moved Peterson to kiss his Brodsky on the side of the head as they left the courthouse.
But the Illinois Third District Appellate Court six months later said Schoenstedt erred and reversed his decision.
The weapon in question is an assault-style rifle. Peterson gave it to his son, Stephen, for safe-keeping, but authorities confiscated it three years ago during the investigation of the disappearance of Peterson's fourth wife, Stacy Peterson. .
Prosecutors said Peterson had illegally modified the weapon by shortening its barrel and charged him with unlawful use of a weapon.
Friday's ruling won't have any affect on Peterson's living arrangements. He continues to be held on $20 million bond in the Joliet County Jail on charges he drowned his third wife, Kathleen Savio.
Peterson has not been charged with Stacy Peterson's disappearance, but many suspect he had something to do with it. Stacy Peterson was last seen Oct. 28, 2007.