The gun is more powerful than the chainsaw, going by a robbery in Melbourne where a luxury car, cash and a mobile phone were stolen.

Two thieves demanded cash from a trio seated in a BMW outside a small strip of shops in Reservoir on Tuesday night.

One of the thieves cut one of the men in the car with a switched-off chainsaw, police say.

The three sought refuge in a shop but were followed by the second offender who pulled out a gun.

One of the victims then handed over cash, a phone and keys to the BMW that was parked in Carson Street.

Police say the car was seen on the Westgate Freeway on Wednesday morning about 6.50am, exactly 12 hours after the armed robbery.

One of the occupants allegedly pointed a gun at another driver while the car moved erratically in the emergency lane.