For sheer ranged damage (anywhere in the world, not just in Tempest Keep), no other ammunition does it better. This is the only Epic ammunition in the game, and it's pretty clear why.

Name: Timeless Arrow (or Shell, for firearms)
Type: Projectile
Damage/Speed: N/A
Abilities:

• Adds 53 damage per second. That's like having an increase of over 700 attack power. The next highest ammo, Adamantite Shells, are +43 DPS, so this is far and above the best ammo you can obtain and use anywhere in the game.

How to Get It: And so of course it doesn't come cheap (in either gold or effort). To get these arrows or bullets, all you have to do is buy them-- they're available from Andormu or Nozari in the Caverns of Time. But the first catch is that you've got to be Honored with Scale of the Sands to do it.

Just to get Friendly with Scale of the Sands, you've got to obtain Vials of Eternity, so that means that you've got to beat both Vashj and Kael in SSC and the Eye. Yup, if you just stopped reading right now, I can't say I blame you.

And after that, I believe the only way to gain Scale of the Sands reputation is to run Mount Hyjal and get rep from kills in there. Work your way to Honored, and you'll be able to buy this ammo. And the last catch on this one? Each stack of 200 is a whopping 2g a piece. Unless you're overflowing with money (and then again, if you've made your way to Mount Hyjal, maybe you are), this is strictly raid boss ammo only.

Getting Rid of It: Vendor will give you 25c back for them if you decide you'd rather not pour out DPS like a Hefeweizen at Oktoberfest.