Billy the Koch, fastest collector in the West

Bill Koch has almost finished building his own Wild West town in Colorado (Rex Features)

If you've heard of David and Charles Koch, the billionaires famous for bankrolling conservative causes in the US, including the Tea Party movement, you may not know about the other brother, Willliam.

The less prominent Koch (they say "coke") is poor (he's worth £2.5bn) compared to his brothers. The men behind Koch Industries and, like Bill, the sons of oil industrialist Fred C. Koch are jointly worth £30bn. But Bill has found a rather more peculiar way to invest in his vision of old America.

The Denver Post reports this week that Bill Koch has almost finished building his own Wild West town in the middle of his remote Colorado ranch. Its 50 buildings include a saloon, a church, a firehouse, a train station, and even a prison.

Don't expect ever to see it, however – Bill has no intention of opening his theme park to the public. He plans to survey it alone from from a giant hilltop mansion.

When he's not on site, presumably in his finest chaps and spurs, Koch – like his brothers – throws money into politics, including a million dollars to Mitt Romney's presidential campaign. He's a sailing nut, having won the America's Cup in 1992 on a yacht called America, and has had three wives.

But Bill is happiest when collecting stuff, particularly relics of the Wild West. His personal museum includes Jesse James' gun, Wyatt Earp's vest, Sitting Bull's rifle, General Custer's flag and a photograph of Billy the Kid that cost him £1.5m. Where can he possibly keep it all?