Paintball gun

The paintball gun is a handheld weapon created on Earth that fires small paintballs as projectiles. Wilf used the weapon to attempt to blind a Dalek by shooting it in the eye stalk. However, the device was useless when the Dalek vaporised the paint, clearing his vision again. But it was destroyed by Rose Tyler with a better gun prompting Wilf to ask to swap weapons (DW: The Stolen Earth)

Behind the scenes
The marker, appears to be a Tippmann 98, or one of its variants, and a non-original barrel.