Explosive Rounds take half of whatever the total damage of the shot and applies it to the area around the target. It's basically free damage to whoever is standing close to your target.
The only drawback is if you needed the rest of that headshot damage to finish off someone and proc a headshot kill perk.
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That's what I thought. It always seemed like I was doing the same dmg as when I'm not using ER. Seems like it's a more pve centred perk. Found in pvp it was taking an extra shot to down the enemy.