The incident happened just before 9 p.m. at the drive through of The Barn in Holtsville on March 12.
Authorities say the suspects ordered a soda, and then one of them got out of the van holding a rifle.
They took cash from the store and fled the scene.
Suffolk County Crime Stoppers offers a cash reward of up to $5,000 for information that leads to an arrest.
Anyone with information about this crime is asked to call anonymously to Crime Stoppers at 1-800-220-TIPS.