From WoWWiki

The ammo type for a Gun. Better bullets or shots have better damage bonuses. Pellets, Pebbles, Shells, Shot, and Slugs are all types of bullets. One bullet stack includes 200 bullets.

Bows and Crossbows use Arrows.

Bullets

Bullet	Level	DPS	Source
[Light Shot]	1	+1.5	Vendor - 10
[Flash Pellet]	6	+2.0	[6] Sharing the Land
[Crafted Light Shot]	5	+2.0	Crafted - Engineering - 0
[Heavy Shot]	10	+3.5	Vendor - 50
[Smooth Pebble]	13	+4.0	Stone golems in Gnomeregan and Darkshore
[Crafted Heavy Shot]	15	+4.5	Crafted - Engineering - 75
[Solid Shot]	25	+7.5	Vendor - 3
[Crafted Solid Shot]	30	+8.5	Crafted - Engineering - 125
[Exploding Shot]	35	+9.5	[35] Stealing Supplies
[Hi-Impact Mithril Slugs]	37	+12.5	Crafted - Engineering - 210
[Accurate Slugs]	40	+13.0	Vendor - 10
[Mithril Gyro-Shot]	44	+15.0	Crafted - Engineering - 245
[Ice Threaded Bullet]	51	+16.5	Vendor - Alterac Valley at Honored Rep 60
[Thorium Shells]	52	+17.5	Crafted - Engineering - 285
[Rockshard Pellets]	47	+18.0	Lord Roccor, Blackrock Depths
[Miniature Cannon Balls]	56	+20.5	Cannon Master Willey, Stratholme
[Impact Shot]	55	+22.0	Vendor - 16
[Fel Iron Shells]	57	+26	Crafted - Engineering - 310
[Ironbite Shell]	65	+32	Vendor - 30
[Halaani Grimshot]	66	+34	Vendor - Halaa if your faction is in control. No longer limited.
[Felbane Slugs]	68	+37	Vendor - Honor Hold at Revered 50
[Hellfire Shot]	68	+37	Vendor - Thrallmar at Revered 50
[Adamantite Shells]	62	+43	Crafted - Engineering - 335
[Mysterious Shell]	70	+46.5	Vendor - Deadwind Pass/Shattrath City at Revered with Violet Eye 1
[Timeless Shell]	70	+53	Vendor - Caverns of Time at Honored with The Scale of the Sands 2

Bullets
Vendor	Light Shot • Heavy Shot • Solid Shot • Accurate Slugs • Ice Threaded Bullet • Impact Shot • Ironbite Shell
Crafted	Crafted Light Shot • Crafted Heavy Shot • Crafted Solid Shot • Hi-Impact Mithril Slugs • Mithril Gyro-Shot • Thorium Shells • Fel Iron Shells • Adamantite Shells
Drop/Quest	Flash Pellet • Smooth Pebble • Exploding Shot • Rockshard Pellets • Miniature Cannon Balls
Faction	Halaani Grimshot • Hellfire Shot • Felbane Slugs • Mysterious Shell • Timeless Shell

Equipment and Weapons
Armor	Cloth • Leather • Mail • Plate  –  Cloak • Necklace • Ring • Trinket • Shield
Armor sets	Druid • Hunter • Mage • Paladin • Priest • Rogue • Shaman • Warlock • Warrior
Melee weapons	Axe • Dagger • Fist • Mace • Polearm • Staff · Sword One-handed • Two-handed • Main hand • Off hand • Dual wield
Ranged weapons	Bow • Crossbow • Gun • Thrown • Wand  –  Arrow (Quiver) • Bullet (Ammo Pouch)
Relics	Idol • Libram • Totem • Spellstone/Firestone