Just wanted to post another approach to this. Skill challenge. This gives you a loose handle for a sniper.
Scenario: Your team has taken position in a log cabin. You are protecting a witness, and just got intel that hostiles have discovered your location, and are approaching rapidly. Air lift is 10 minutes away. Radio out.
Your sniper takes the roof top, attaches his scope, and begins to scan the perimeter. Hostiles are advancing cover to cover. So you enter a skill challenge. Two of your allies secure the witness and put up barricades, one companion with binoculars assists with spotting and watches your back.
You start making perception checks to spot the enemy, you or your spotter can make insight or history to decipher their tactics. The enemy might be using smoke grenades for cover. After a successful perception, you make a ranged basic attack, range is nebulous and irrelevant. You can accumulate damage to see if it's enough to take a creature down before X number of rounds go by and the enemy is at the door. Meanwhile some athletics checks are made for barricades. Maybe some allies set a booby trap near the entry way with a thievery/arcana check, which all contribute to the success/failure of the skill challenge (have to keep everyone busy, not just the sniper).
Depending on the success/failure of the skill challenge, the final encounter when the enemy busts in the door will essentially be a level lower or a level higher. Using minions, you could do a more graduated success level. Did the sniper take 4 shots? there are 3 minions instead of 6, and maybe another enemy took it to the kevlar, so he starts the encounter down some hit points.
You don't really need a combat mechanic to make attacks at long range. That feels too ripe for exploitation. When you tie it to a skill challenge, the sniper is still going to be doing their job, reducing enemy numbers, taking down some red shirts, without the complexities of a new mechanic.
And depending on how well you like the encounter you run, you can always modify things for the next skill challenge, to improve the overall experience, and you haven't tied yourself to a single mechanic you might regret later.