Payday 2 anarchist and crew chief

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.

No - crew bonuses do not stack - at all. In the instance two or more of your crew members have an ability that provides a crew bonus, these do not stack. However, there are a few caveats;

• Fast Learner (ace) provides a crew experience bonus, this is separate from Fast Learner (not ace) which provides a personal experience bonus. These two bonuses are separate bonuses, meaning if every member of your crew has one point in Fast Learner and one of your crew members has it aced, every crew member will receive a double experience bonus (a personal bonus from having one point in Fast Learner, and a crew bonus from the crew member that has it aced). If two or more crew members have Fast Learner aced, this bonus does not stack.

• Endurance (ace) provides a crew stamina bonus, this is separate from Endurance (not ace) which provides a personal stamina bonus. This works in the same way as Fast Learner.

• Abilities like Leadership (both ace and not aced) which provide crew bonuses at both levels do not stack. If multiple members of your crew have one of these abilities there is no additional bonus provided to the crew (they will, in the instance of Leadership, receive a 25% bonus for the first tier or a 50% bonus for aced Leadership)

This means, if you're building up a crew that is going to run heists together constantly, you'll want to avoid doubling up on abilities like Leadership but may want to each expend a point in abilities like Endurance or Fast Learner.