Piercing Shot is a Commando ability which boosts the power of sniper rifles and causes them to inflict substantially more damage with every shot. However, note that it is impossible to get a critical hit while this ability is active. Be careful when using it in stealth as a yellow crosshair lights up above your head which can be seen by everybody. Also the duration of the ability is very short, so make every shot count!
Using level 5 with the fast sniper rifle is a deadly weapon even at close range.
Using a headshot sniper rifle such as the SVD or the M95 you can hit very high in the head..... as much as 80 depending on the target.
Piercing shot helps to steady your aim so it can be useful even at long range.
Activating piercing shot at the same time as shooting will still include the bonus damage although it seems piercing shot has not activated yet. This is especially useful with fast-firing sniper rifles as it is possible to count one more shot in the short period piercing shot is activated.
Activating Piercing shot, with MK.I Badboy/Panzerhunter 39 and Workbench modification Packin' III, proves to be very deadly.