I'm betting it was this

Essentially a RM sniper engaged the bridge of the ARA Guerrico with a L42A1 forcing the captain to take cover!
Leach was armed that day with the right weapon for that job: the L42A1 rifle. A conversion of the Lee–Enfield No. 4, Mk. 1(T), the L42A1 was chambered for the 7.62x51mm NATO cartridge and mounted the 3.5-power No. 32 scope. Lying on the table on the second floor, the Sergeant Major placed the post of his reticle on the approaching ship’s bridge. By then, Guerrico was once again facing the channel and closing on King Edward Point. A moment later, as the other Royal Marines began hammering away at the ship for a second time, Sergeant Major Leach began firing carefully aimed shots at the vessel. He directed his opening rounds at the five windows across the front of the bridge. At this point, only Captain Alfonso, the helmsman, and the quartermaster were manning that station as glass began to shatter. The three men were forced to crouch down behind ship’s structures to avoid being struck by the rapid succession of accurate shots coming from Leach’s sniper rifle.

See link for the full article
http://www.warhistoryonline.com/guest-b ... gan.html/2
A very impressive action fought by some very brave marines! Also its pretty impressive that the first British, adopted, Magazine Loading Action invented the late 1800's made it through to 1982 still delivering the goods.
I wonder what distance he was firing over and exactly how effective the fire was, does anyone know any more information?