A Canberra teenager who held up a supermarket at gunpoint in front of witnesses including a nine-year-old girl says he had a shotgun stuck in his own face just days earlier.

Alexander Cameron has pleaded guilty to the aggravated robbery of the Hackett IGA last year.

Cameron and a co-offender stormed into the supermarket on the night of March 8 with gloves and face coverings. Cameron, 18, was armed with a .303 rifle he later said was not loaded.

The defendant kept the weapon in a ''ready to fire'' position while the pair forced cashiers to hand over $1300 cash.

According to a statement of facts tendered in the ACT Supreme Court, Cameron said to one of the employees: ''Do you want me to shoot you?''

Another of the workers, who had known Cameron for about five years, recognised his voice.

The employee met Cameron by chance the day before, when the defendant asked if he could rob the supermarket while the employee was on the checkout. The employee refused.

As Cameron and his co-offender fled the store and got into the get-away car, a stolen Subaru Impreza, the gunman pointed the weapon at passers-by and said: ''Don't look or I'll f---ing kill you.''

Six customers were in the supermarket, including a nine-year-old girl and her mother.

In court on Monday Cameron said he could ''only imagine how terrified [the mother] was for the safety of her young daughter''.

In a statement read out in court, a cashier spoke of the traumatic impact of the robbery.

''Some days I just don't want to go to work … loud noises and shouting scare me at work,'' he wrote. ''After serving someone who doesn't look friendly my hands shake and I don't want to be behind the counter. I just want to go out the back and calm down.''

He described a comment from one of the robbers - ''Just shoot him'' - as the most traumatic moment of his life.

Cameron told the court he committed the crime so he could pay back money owed to dangerous people.

Cameron said less than a week before the robbery someone pointed a shotgun at his face and demanded the money back, warning him he knew where he lived.

A police witness told the court the defendant only volunteered the information about the threat and shotgun incident during a third interview with police.

The court heard Cameron began using cannabis and drinking alcohol at an early age, and by the time of the robbery he was consuming the drug ''ice'' daily.

Mr Hopkins asked Justice John Burns to consider deferring his client's sentence, giving him time to complete a residential drug rehabilitation program.

But the judge said he would hand down his decision next month.