Every angler has a cracking fishing tale, but one Perth fisherman has probably the greatest yarn of all time after reeling in a gun from the Swan River on Friday.

Khao Nguyen was fishing under the Narrows Bridge in South Perth, when he jagged something on his line.

A Perth fisherman got a surprise when he caught a gun in the Swan River. Credit:Seven News Perth

"I was hoping to catch something and to my surprise I caught a gun. Normally you don't expect to catch something like that," he told Seven News Perth.

"I cast out halfway through the channel and I start to reel it in slowly, and that's when I hooked the gun, but I didn't know it was a gun, I thought it was a fish."