At 9.20pm on Monday, July 26 two men went into the Co-op shop in South Street, Tarring, Worthing, and demanded money from the staff.
One of the men had what appeared to be a handgun and the other man produced what he said was a canister of CS spray. Neither the gun or the can were set off.
Detective Sergeant Jason Turner said: "Detectives are conducting a thorough investigation into this serious offence.
"If you saw anything suspicious in the South Street or St Dunstans Road area at the time of the robbery or have any information please report it."
One member of staff, a man in his late 20s, was punched in the face and body. He was treated at Worthing Hospital.
The men took about 200 cash and left through the rear entrance into St Dunstan's Road.
The man who held what he said was a CS spray canister is described as being in his 30s, and about 6ft 5in tall.
The second man, who held what appeared to be a handgun, was aged about 30, 6ft and of stocky build.
Both wore dark balaclavas, dark jeans or trousers, and trainers.
Anyone with information should contact Sussex Police on 0845 6070 999 quoting serial 1868 of 26/7 or anonymously to the independent charity Crimestoppers on 0800 555111.
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