Payday 2 – Best Ways to Improve the Gang’s Safe House

Steam user Faildozer has written a very detailed guide, " PAYDAY 2: The Numbers" ("taken from the game files"). Here is what he has to say about ammo packs, supporting your theory:

The ammo bag starts out with 4 uses, some further info about how usage is counted by the game:

• It's not split between primary and secondary ammo; there's just one pool of ammo that weapons draw from
• How much ammo drawn from the bag is based on a percentage of how much ammo you've used so an empty AK draws just as much as an empty M308 which draws just as much as an empty Crosskill and so on
• A default bag holds enough ammo for exactly 4 full refills (400% ammo if you want to think of it as a percentage)
• The Enforcer skill that increases the capacity of the ammo bag gives you 50% more ammo per bag (so 6 full refills/600% ammo)
• The game will always prioritize filling up your secondary first before your primary, so if both your primary and secondary are completely out of ammo, and the bag has exactly 1 full refill left (or 100% ammo) then your secondary will be refilled completely and your primary will still be empty

Medic bags do get used up after 2 uses (without upgrades). Similar to Left4Dead, you die for good if you go down too often, but healing from a med kit resets this counter. So saving them for teammates that went down multiple times already might be worth it.

So it turns out in the install directory of the game, for me, E:\Steam\steamapps\common\PAYDAY 2 there is a PDF which details the game. It's got a handy page on the stats!