From the question How does auto-pickup work?, we know that ammo is shared between players in co-op. How does this sharing work? Does each player get the full amount of ammo, or is it divided between them? Does it make a difference if one player already has full ammo of the type being picked up?
I'm also interested in the sharing of money and Eridium, but I believe those are covered in How does experience work in co-op?
Each time you pick up ammunition, every member of your party receives ammunition as though they had picked it up. This is different to the way gold and eridium is shared and there is no indicator to show that you have gained ammunition as a result of one of your party members picking some up.
The below is quoted from one of the discussions as a result of testing the behaviour of this mechanic:
When you or your friend collect ammo it goes to all players. It isn't like 5 bullets go to each player or some crap like that. Everyone gets every ammo drop ever. Yes it works. It is the same for cash and eridium...the only indication of you getting ammo is if you are in your backpack at the time watching it.
To add to this answer, grenades and rocket launcher ammo are an exception - only the player who picked them up gets them. This is probably to avoid over-powering grenades and rocket launchers (as most of them are designed to have limited ammo).
I'm pretty sure the ammo is for whoever picks it up only, unless you're full. If you've got max shotgun ammo for example in single player you can't pick it up, but in multiplayer it'll let you because then it goes to someone who needs it. I don't know how it decides who gets it. It can't share it the way it does cash. My buddy snipes a lot and always runs to some high point to do it so he's not picking up anything and he runs out of ammo even though I've been running around and picking it up for me. If it shared like it does the money, he wouldn't run out like that.
