Officers were last night hoping to match the weapon to the killing.
Mr Collins died after he was shot at point blank range on Apr 9, 2009, at an arcade business which he had been running beside his father’s pub, The Steering Wheel, at the Roxboro Shopping Centre complex.
Gardaí believe he was murdered after his family gave evidence against members of the McCarthy-Dundon criminal gang.
The gun was discovered at around midday on Saturday during a community clean up near Young Munster’s rugby club, detectives said yesterday.
The weapon was revealed after a grass-cutting machine cleared an overgrown area near railway tracks in an area known locally as Mill Lane.
Gardaí confirmed it was discovered in the same area where a getaway Mercedes car, used by Mr Collins’ killers, was found abandoned and on fire immediately after the 2009 fatal shooting.
In May 2010, the gunman James Dillon, 26, of no fixed abode, was sentenced to life in prison for the murder.
However, gardaí are actively pursuing others as part of the murder probe — including those who ordered the murder hit and a man who was seen in the getaway car with Mr Dillon moments after the shooting.
Earlier this year, the Collins family fled Ireland due to sustained threats from the gangs.
“The gun has been sent to the garda ballistics laboratory in Dublin and will undergo testing to see if it is the actual gun that killed Mr Collins,” a Garda source confirmed yesterday. “The gun will also be tested for fingerprints.”
Gardaí will also see if they can cross-check the gun against spent shell casings found at the scene of the murder of Shane Geoghegan who was shot dead in a case of mistaken identity in Limerick on Nov 9, 2008.