The two were reportedly seen on CCTV arriving at Applegreen services on the A23, Crawley, in a silver car at around 4.15pm on Tuesday (June 12).
Sussex Police say one of the suspects distracted the till staff while the other man took two cases of beer and the pair left.
The staff member chased after the suspects and one of them showed, what was believed to be, a gun.
Detective Constable Adam Tidy said: “We would like to speak to anyone who has information on these men.
“We believe this to be a targeted and isolated incident.
“We would also like to speak to anyone who may have information on the vehicle the pair arrived in.
“In particular, if you were driving along Crawley Avenue towards Cheals roundabout from 4.15pm until 4.20pm we are appealing for you to review the footage. o you have any dash cam footage of the silver car?”
You can report information online or by calling 1014 of 12/06.
Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.