Yeah, they really are messed up. This is how it works for regular weapons in Co-op right now and possibly Competitive too:
AK: 120 (80% of *WMA)
G-MAL: 30 - 45 (25 - 37,5% of WMA)
M9: 64 (50% of WMA)
Dragon Sniper: 5 - 15,**17 (20 - 60% of WMA with MA mod, 25 - 75% without MA mod)
I won't mention sidearms, because they seem okay IMO (and I can't even remember their pickup amounts ).
So, as you can see, AK gets HUGE amount of ammo when compared to others. They should be balanced so that each weapon gets the same (or similar) % amount of the weapon's max ammo.
This is how the ammo pickup should work for both Co-op AND Competitive:
AK: 75 OR 90 (50 - 60%)
M9: 64 (50%, same as before)
G-MAL: 45 OR 60 (37,5 - 50%)
Dragon Sniper: 10 (40 - 50%, depending on the mod)
The reason I have more than one option for AK and G-MAL, is because of the clip size mod. The 90 shots for AK, and 45 shots for G-MAL "work" better with the Clip size mod (Both AK and G-MAL get 45 shots in the clip, at least for Competitive). So when you pick up an AK, you get two clips, and for the G-MAL, one clip.
However, G-MAL won't get much chance in the amount of ammo picked up if it's 45, so that's why I have 60 as another option (and it's more balanced with other weapons too). But then again, G-MAL is very accurate, so you miss less when compared to other weapons.
The 64 shots for M9 is perfect, and should not be touched at all.
And finally, the Dragon's random amount of ammo should be changed to something like 10, and possibly extend it to 15 if max ammo mod is installed, but I don't find it necessary. It's more balanced that way, and the amount also stays the same as when you normally pick up a Dragon from the ground.
*Weapon's max ammo.
**I remember sometimes getting as much as 17 shots from a Dragon on the ground. It might have been someone else's though.
While I certainly agree that this needs to be fixed in Co-op, in a competitive sense I feel it could add some balance. For example, if less ammo is picked up with M9s, it would certainly turn people off from using RoF since they would less and less ammo forcing them to use weapon expert or pick up a different gun. However, I am just speculating and bring out some ideas.
Oh, and when you finish your T-bolt bullets, and see a brand new flashing T-bolt and say: "awesome 5 NEW BULLETS", Nope guess what? You only get 3...
From what I notice, the T-bolt just fills back up to 5 no matter if you have 0-4 bullets.
Really? Got to check that one out, but...
I picked up an AK with 164 bullets in Classic mode, once.
Probably belonged to a person that had a Max ammo stat hat.
Stat hats in Classic mode = Another illogical decision by Naughty Dog.