All the simple monster re-skinning in 4E made me think of a simple yet effective reskin I used in a 3.5 campaign a few years back. My players were tasked with stopping a master assassin who had been killing political figures by magically sniping them from incredible distances away.
When they eventually tracked him down, he was on a rooftop (the party was on the ground), wielding what looked like a heavy crossbow minus the bolt and bowstring mechanisms. He opened fire on the party, shooting shimmering magical shots with unfailing accuracy. Each of his shots did 1d20+10 damage, which made for some great reactions around the table...
Me: (after rolling and confirming a hit) Okay, that's a hit so... *rolls d20*
Player: What? He gets two shots?
Me: No, this is his damage roll.
Player: What?! HE DOES d20 DAMAGE??
The players scattered for cover but he still shot them with pinpoint accuracy, no matter how low I rolled, before making his escape. Afterwards the shaken and nervous party revised their "yell, charge and bash things until they die" methods to properly combat this terrible challenge.
Seems unfair? After all, a weapon that does d20 damage? That's ridiculous!
Until you consider that the "sniper rifle" is just a slightly modified 9th level wand of Magic Missile.
For flavor, I made it do 1d20+10 instead of five 1d4+1 missiles, which upped the damage range to 11-30 dmg instead of 10-25 dmg, but balanced it by making the shooter only able to target one creature.
The never-miss aspect of Magic Missile is what made the trick really work though, and convey the feeling of a master sniper with amazing skills. I rolled attack rolls for the assassin, but this was simply for show and to mess with their heads - nothing wreaks havoc with a metagamer's head like seeing the DM roll a 2 and still hit a rogue taking cover behind a tree. ;)
If you wanted to extend the rifle theme further, you could have the shooter change "clips" by ejecting the old wand and inserting a new one for a fresh 50 shots. Or have the wand utilize the Enlarge Spell feat to double its range to ~400 feet of sniping accuracy.