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 acquaintance at the front of a property about 11pm.
Police said the man allegedly threatened the man with a firearm, before stealing cash, wallets and a mobile phone. Investigating officers later searched properties looking for the weapon, before discovering it in Wanneroo.
Sen-Sgt Burnett said it was a good outcome for police.
‘Getting a gun off the streets is always good,’ he said. ‘If anyone knows of any illegal firearms, please let us know.’
The defendant was charged with armed robbery, refused bail and will appear tomorrow in Stirling Gardens Magistrates Court.