A 'turkey shoot' is a
metaphor for an easy target for
small arms fire. Similar to '
sitting duck' or
shooting fish in a barrel. I think, though, that the
metaphor refers to a literal turkey shoot, bringing to mind a few men with
shotguns unloading them into the crowded penful of spring turkeys.
Specifically, an example would be the use of the L-shaped ambush. The men in the fire team are arranged in an L along the curve of a road or path. The short line of the L contains the machine gun, which fires at the front of the oncoming infantry or vehicles, while the long line provides crossfire. They wait until the target is in the 'kill zone', and then everybody opens up at once, usually cued by the machine gun.