The police helicopter was scrambled and golfers cleared from the greens at Ifield Golf Club following the alert at around lunchtime.
Eyewitnesses said the clubhouse was cleared and closed while police carried out a search for the gunman and the police helicopter circled overhead.
A spokesman for Sussex Police said later that the armed man was ‘legally shooting.’
He said: “Police checked a report of a man with a gun in a field at Ifield on Thursday afternoon.
“It was found that he was legally shooting.”