With the Battle Rifle
equipped, it takes exactly four headshots
with the weapon to kill another player with full shields. The way most experienced players pull one off is to BR three times in the chest, then pull the crosshair up for a headshot. This maximises effectiveness as in compensates for the BR spread, as a chest shot is a bigger target than the head itself. This is referred to as a Four Shot. This can be easy or difficult depending on the other player's skill, cover and footwork.