You can't hide the facts from The Associated Press:
In this image made available in London, Thursday Feb. 28, 2008, Britain's Prince Harry mans a 50mm machinegun, at an observation post close to Forward Operating Base Delhi, in Afghanistan, on Wednesday Jan. 2, 2008. ...
Fifty millimeters? That's two inches. .50 maybe? Bit small for that isn't it? A .50 BMG cartridge is almost six inches long. Betcha somebody at the AP either saw "7.62x51 NATO" on the press handout and helpfully "corrected" it, or saw "machine gun" and threw in "50" just because he vaguely remembered the number "50" has something to do with machine guns, and then "mm" because he vaguely remembered millimeters have something to do with guns in general.
Best and fondest wishes to Prince H, by the way, and to all who serve with him. Finest traditions and all that. Makes a man proud to have, through no virtue of his own, been born in a country where people speak English. Whatever's wrong with his dad, I guess it's not inheritable. I hope they don't make him go home now the story's leaked.