The soldier on the left has his rifle strapped to a wooden device which incorporates an angled mirror at the top (and certainly another at the bottom), with probably a wire going to the trigger, enabling him to aim and fire without lifting his head above the surface of the ground.
It appears he is being aided by a spotter behind him. I don't know the location, original source, photographer, or context for this. The hats suggest perhaps Gallipoli? Anyone know?
Addendum: A hat tip to Bogong, who found the original image at Australian War Memorial:
Gallipoli, Turkey. 1915. A British Admiralty official photograph of a sniper using a rifle with a periscopic sight while an observer checks the results with a trench periscope. The periscopic rifle was invented by an Australian soldier. The men belong to the 2nd Light Horse Regiment and the place is probably Quinn's Post. Left to right: 274 Sergeant Ernest William Crain; unidentified; 313 Trooper Arthur Snowdon Demaine; Lieutenant Joseph Burge (killed in action 7 August 1915).