Telford College music technology student Tom Walker, 20, was magnet-fishing with a friend under the rail bridge near The Dana in Shrewsbury when he made a 'shocking' discovery.
Among the scrap metal and trolleys the duo would usually pull from the river was what Tom first thought was fishing equipment, but turned out to be part of a rifle.
Tom explained: "I couldn't believe it, I thought it must be something people used for fishing - never thought it would be an actual gun.
"When I realised it was, I assumed it had to be an air rifle or something but when I started to wipe the dirt off I realised what it was.
"It looked like a 7.62mm military-style rifle."
The pair called the police non-emergency number for advice, and officers told them to take it down to the nearest police station.
Tom added: "I told them that I'd found a gun, but I think they thought it was nothing. I think they were shocked.
"I've found all sorts in the river, mostly trolleys and hedge trimmers and random bags of rubbish. But they go in the bin, we try and take as much rubbish out of the river as we can.
"The gun is definitely the most worrying thing I've ever found."
A spokesperson for West Mercia Police confirmed the find: “On Sunday September 18 at around 6.10pm we received a report that two members of the public had discovered what appeared to be part of a rifle in the River Severn in Shrewsbury.
“The object was handed in to Monkmoor Police Station on the same day, where it was seized by officers."
Police are asking residents who make similar discoveries to inform them immediately, so that appropriate action can be taken.