Daniel Snyder’s private equity firm has sold Johnny Rockets, its oldies-themed burger-and-milkshake chain, for an undisclosed amount after six years of ownership.

An affiliate of Sun Capital Partners purchased the 300-unit chain this month from Snyder’s Red Zone Capital Management, based in McLean, according to a spokesman for Sun Capital.

Red Zone managing director Norm Chirite declined to comment on the sale. Red Zone hired North Point Advisors earlier this year to find a buyer. North Point did not return telephone calls seeking comment on the deal.

Red Zone bought Johnny Rockets in 2007 from Centre Partners Management and Apax Partners, two private-equity firms. According to Nation’s Restaurant News, the Johnny Rockets chain operates in 30 states and 16 countries.

Washington restaurant broker Tom Papadopoulos said he did not fully understand the strategy behind Snyder’s purchase of Johnny Rockets.

“I think he was going to put it in his Six Flags parks,” Papadopoulos said, but the purchase “made no sense. It’s not his business.”

The company has restaurants in 17 Six Flags parks and on 11 Royal Caribbean cruise ships, according to Nation’s Restaurant News.

Johnny Rockets is one of several high-profile investments, including movies and amusement parks, that Snyder, who is majority owner of the Washington Redskins, has made over the years. Those investments have been made through partnerships or Red Zone.

Last fall, Red Zone sold its interest in Dick Clark Productions to Guggenheim Partners at a price that was reported to be double what Snyder had originally paid.

Snyder’s other investments have included a big ownership stake in Six Flags amusement parks. He took control of Six Flags in 2005 after a bitter proxy fight and became its chairman. But the company collapsed under $2.4 billion in debt that Snyder inherited from Six Flags’ previous owners, and the company filed for Chapter 11 protection.

Working through a partnership called First and Goal in 2006, Snyder, former Six Flags chief executive Mark Shapiro and NVR founder and Redskins share owner Dwight Schar made a two-year deal with actor Tom Cruise’s production company to pay $3 million to $10­ million a year for the opportunity to finance Cruise’s film projects.

All three were listed as executive producers on “Valkyrie,” the Cruise film about the real-life plot to kill Adolf Hitler.

Forbes magazine lists Snyder as having a net worth of $1 billion.