Firing Arcs refer to the width of a weapon's Overwatch radius (when an overwatch talent is used for burst fire) or Full-Auto radius (supposing the weapon supports a fully automatic fire mode.) Talents, buffs, weapon mods can all increase and decrease this width. It is measured in standard degrees of a circle, totaling 360 degrees.
Below is shown a 75 degree arc which is the default of a UAR.
