From Maclean’s (Military Magazine):
Lying low beside the rifle, his stomach touching the ground, Cpl. Rob Furlong concentrated hard on his breathing. In, out. In, out. In, out. Deep, but not too deep. Slow, but not too slow. The tiniest twitch — a heavy exhale, perhaps, or a breath held one second too long — could jerk his weapon ever so slightly, turning a sure hit into a narrow miss. In the sniping world, where one shot should always equal one kill, steady breathing is just as crucial as steady aim.