Officers last week charged a 28-year-old Embleton man and are investigating another man over the June 21 incident.
Senior-Sergeant Brad Burnett said police will allege the pair were riding in a silver Toyota Prado on Dunmore Circuit in Merriwa when they saw the alleged victim, who was known to them, at the front of a property about 11pm.
He said the alleged offender threatened the man with a firearm, before stealing cash, wallets and a mobile phone.
Investigating officers later searched a number of properties looking for the weapon, before police allege they discovered the gun at a Wanneroo residence.
Sgt Burnett said it was a good outcome for police.
‘Getting a gun off the streets is always good,’ he said.
The accused was charged with armed robbery, was refused bail and will next appear in Stirling Gardens Magistrates Court on July 9.