Okay all, this should be common sense, but here goes.. Shuriken are NOT ammunition.
There are two references to shuriken being ammo, and I quote, "shuriken are treated as ammunition for the purposes of drawing them, crafting masterwork or otherwise special versions of them, and what happens to them after they are thrown."
To be honest, the last part has me confused a bit, but they are not on the magical ammunition breakage chart.
"Magic Ammunition and Breakage: When a magic arrow, crossbow bolt, or sling bullet misses its target, there is a 50% chance it breaks or is otherwise rendered useless. A magic arrow, bolt, or bullet that successfully hits a target is automatically destroyed after it delivers its damage."
And the spell magic weapon says "Treat shuriken as projectiles, rather than as thrown weapons, for the purpose of this spell."
Meaning you can do 50 at a time (because they're tiny, I guess)
In both cases it clearly says 'treat as' which means they are not ammunition.
They are not on the ammo chart, they are in the Exotic ranged weapon chart.
This means they are ranged (Thrown) weapons much like a dagger that you can't use to melee with.
They don't break when you throw them, like any other thrown weapon. Which, in my opinion, is a no-brainer anyway.
Enchanting them, unlike the spell magic weapon, is a 1 on 1 deal, so if I were you, I'd put returning on them. I enjoy using my +3 Flaming shurikens of returning every chance I get.
I hope this has been helpful to everyone.