A 25-year-old member of staff was working alone in Grande Wine Store, Palmyra Road, Gosport, when the gunman walked in and demanded cash.
Manager Carla Frankland praised the swift actions of the shop assistant who opened the till and ran to the back of the shop and deadlocked the door.
He watched the masked robber on CCTV take cash and leave before he went back in use the phone and call police.
Miss Frankland said: ‘The young man was very calm when it happened and did exactly what he was meant to do.
‘The police were brilliant. They kept him on the phone until the armed response team arrived.’
It happened just before 10pm on Sunday, March 27.
Police have launched an appeal for witnesses – Det Con Julie Way, the officer in charge of the case, said: ‘I’m especially keen to hear from a man and a woman who used the store shortly beforehand and who may have seen what happened.
‘I’d also like to hear from anyone who may have found discarded clothing in the area.’
The robber was described as being white, between 6ft and 6ft 3ins and of skinny build.
He was wearing a black padded raincoat with the hood up, dark trousers and black shoes, ski-style gloves and a mask.
Anyone with information should contact Det Con Julie Way on 101 quoting 44160119412.
Alternatively call Crimestoppers anonymously on 0800 555 111.