History: During the the middle of the Clone Wars, Kaminoans started cloning clone gunners. Clone gunners were used in all sorts of tasks. They fought along side regular clonetroopers.
About: This particular clone gunner is firing a laser machine gun.
Most often, if clone gunners had to change their position, they would have two other clones with them. (One to carry the main body of the gun, one to carry the barrel of the gun, and one to carry the ammunition.)
Clone gunners did almost everything. They piloted walkers, shot cannons in space, and even worked on machinery.