Simon Douglas, 52, who murdered a fellow prisoner in England in 1993, was out on licence when he carried out the assault and robbery at the shop in Lochavullin Drive, Oban, on August 13, 2019.
He approached a member of staff, who was working on a till, pulled up his shirt, showed the butt of a gun and threatened to shoot him.
Jurors heard that the weapon was a BB gun.
At the High Court in Glasgow, Douglas, from England, who has 57 previous convictions, denied the offence but was convicted by a jury after a five-day trial.
Judge Lord Braid told Douglas: 'You have an appalling record and were on licence at the time.'
Douglas was released from prison in October 2018 by the parole board and ordered to stay in a hostel in England. However, he left there and toured around Scotland camping out.
On the day of the robbery he took a bus from Fort William to Oban, bought the BB gun, carried out the robbery and then got a bus back to Fort William.
In evidence, Lawson claimed that he did not have a gun on him when he went into Marks and Spencer. He said that he had been asked by a gangland figure called Chris from Oban to collect an £800 debt from the store assistant.
He was asked by his defence counsel Geoff Forbes: 'Did you expect him to take the money from the till?' and Douglas replied: 'No, I was focusing on taking the money and leaving.'
Douglas was asked what Chris's surname was and replied: 'I have to refuse to name him as I would be putting my life at risk.'
Prosecutor Stewart Ronnie asked Douglas why he had dumped the jacket he was wearing and bought another after the robbery and was told: 'To change my appearance. The world of my friend Chris is a dangerous murky world.'
The jury did not accept Douglas's preposterous story and convicted him of threatening the store assistant with violence, presenting a BB gun at him and robbing him of £500.
Douglas showed no emotion as he was led away to begin his sentence.
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