Mercy rules are for close quarters (within 10 feet) with regular weapons. There are no mercy rules with sniping. That's what mimimum distances are for. I won't pretend to know what they are, as I'm no sniper and have nowhere near enough experience to be able to be trusted in a sniper role with a hot gun. Being a sniper is a lot about knowing when you can or can't take a shot safely, even if you have a perfect shot opportunity.