When a marine respawns, they don't just suddenly get their starting ammo back right? So if they have no ammo and die, they have no ammo when they respawn?

Correct. The only thing that "resets" are the health tokens and the armor tokens. Ammo is not touched after a Frag.

-shnar

And remember that you literally resets armor tokens. So if marine died with more armor then he should have (base 2 + optional skill cards) he drops excess tokens.