TL;DR To avoid getting little or no ammo from a bag, count the ammo points you're missing from your kit (100 ammo points per slot, split evenly among the items in that slot) then pile up an amount of bags that will fully cover it (150 points per bag as of v1.3.6). Make sure no one, not even you, comes near the bags until the pile is finished. When it is finished, you - and only you - may step onto them. Stay there until you finish resupplying.
I haven't tried this in a normal game yet.
Introduction
When you resupply from a bag, ammo points move from the bag to your kit over time, spread evenly across all slots that need refilling. Once a slot has enough points for another item, you receive that item. If the bag is used up or if you leave its range, any "partially filled" items are reset to 0 without giving you the item.
Each slot in your kit is worth 100 points, with those points split evenly among each mag/item in that slot. The currently loaded mag/item counts toward the total, e.g. 8 reserve mags + 1 in the gun = 9 mags, 11.1 points per mag. Thus a patch in most kits is worth 100 points, 33.3 in officer (3 patches), 16.6 in medic (6 patches).
Each bag used to hold about 200 points (two slots' worth) and since 1.3.6 it's been changed to 155-160 in order to make it impossible to get infinite AT rounds by refilling a bag from itself. This fix causes some other problems.
For example, if you use a patch and a rocket in a kit with one rocket, an ammo bag's 160 points are split 80-80 between them and neither item reaches its required value of 100, so you get literally nothing from the bag. You can burn through a lot of bags this way.
Test setup
USMC standard LAT kit on Muttrah City:
- M16A4 with 8 reserve mags + 1 in the gun
- single AT4 rocket
- 2 frags
- 2 smokes
- patch
Based on the math in this post, rocket and patch are 100 points each, grenades are 50 points each, and mags are about 11 points each. (100 points / 9 mags = 11.111... repeating)
I tried to round mag value up to 12 but it gave me inconsistent results so I won't use that. I'll truncate results so 155.555... becomes 155.5.
Results
Emptied 8 mags (out of 9). Did so by firing once and reloading, until the reserve mag count was 0 but the mag in the gun was still full. One ammo bag refilled all 8 mags without disappearing. I emptied mags again the same way, the bag refilled 5 more mags and disappeared. So one bag refilled 13 mags. That's between 144.4 and 155.5 points.
Emptied 1 rocket. One bag refilled it, of course. Then I emptied some mags and the remainder of the bag refilled 4 mags before disappearing.
Emptied 1 rocket + 4 mags. One bag refilled everything and remained, but disappeared when I tried to refill another mag.
Emptied 1 rocket + 5 mags. One bag refilled all mags BUT NOT THE ROCKET!
Emptied 1 rocket + 8 mags. One bag refilled only 7 mags and no rocket.
Emptied 1 rocket + 1 frag. One bag refilled everything and disappeared, for 150 points.
Emptied 1 rocket + 1 patch. One bag refilled NOTHING!
Two bags refilled everything and the remainder refilled 9 mags before disappearing, for a total of 300 points, or 150 per bag.Emptied 1 rocket + 1 patch + 2 frags. Two bags refilled everything and the remainder disappeared when I tried to refill another mag. Total of 300 points plus some amount less than 11.1 points (one mag's worth).
Emptied 1 rocket + 2 frags + 8 mags. One bag refilled only 4 mags and nothing else. The fact that only 4 mags were refilled demonstrates that the bag is split up amongst all slots that need refilling, even if it causes waste.
I wondered what could be done with bags left over after the rifleman resupplied the bag from itself. I spawned N rifleman kits, stood in the same spot pressing G to pick them up and threw their bags so they would end up in the exact same place. Then I picked up all the kits with missing bags and resupplied the bags from the bag pile, leaving only the extra ammo generated out of thin air. This remainder ammo should be 50 to 60 points per bag since a full bag is 150-160 and refilling a slot takes away 100.
One bag's remainder refilled 1 frag and disappeared. 50 points.
Two bags' remainder refilled 1 rocket and disappeared upon trying to refill another mag. 50 to 55.5 points per bag.
Three bags' remainder refilled 1 rocket + 1 frag, and what was left refilled 1 mag before disappearing. 53.7 to 57.4 points per bag. Note: When I piled three bags imperfectly (like I would in a game), they didn't give me that last mag because when I walked into them, the near bags started resupplying slightly before the far ones, thus disappearing a moment earlier as well. This demonstrates how you lose ammo if you don't use the bags at the exact same time.
Conclusions
An ammo bag holds at least 153.7 and less than 155.0 points, assuming everything was perfectly accounted for and no hidden mechanics caused inaccuracies. EDIT: Lowered upper bound according to grenade launcher tests. EDIT 2: There are definitely inaccuracies. One bag is supposed to hold 160 points. It's safe to assume one bag will refill 150, but whether you get any more than that will vary.
The complete or near-complete waste of bags in common situations is a horrible feature by anyone's standards. Having 200 points in each bag was some protection from it because you could use up two whole slots and still resupply from one bag. Now if you use up two slots, you waste up to 100% of the bag depending on what the items are, so it becomes super important to know the details of how ammo works.
You'll often be forced to scavenge multiple bags at once just to get a single item. And that will only work if 1) you use the bags together at the same time, 2) you're extremely careful, and 3) your teammates cooperate by not touching the bags.
Further research
Added this section after doing more tests.
One bag refills 30 grenade launcher frag rounds. GL holds 19+1 rounds. One bag must be less than 155 points.
Ammo box (either kind) refills at least 445 and less than 450 points. Although, with 2 ammo boxes together I was once able to refill 8 slots + 21 GL frags, meaning the boxes must have held at least 452.5 each? Confused.
Light supply crate refills 500 points. Not much more than an ammo box.
Heavy supply crate refills at least 995 and less than 1000 points.
Deployable weapons count as one slot, as expected.
Grappling hook is refillable and costs 100 points like any other slot. Retrieve your rope before resupplying to avoid wasting ammo. (Yes, one breacher can deploy multiple ropes.)
Shotguns behave strangely. The game thinks there's only 1 shell loaded in the gun. So a USMC breacher with 14 + 7 rounds is considered to have 14 + 1 rounds for ammo purposes. As a result, the cost of fully refilling a fully emptied USMC shotgun slot is 140 points instead of 100.
I also tested with ARF scout, which has 28 + 7 shells interpreted as 28 + 1, theoretically 120.7 points to reload a full slot.
But I ran into an inconsistency. One bag reloads 43 ARF scout shells instead of 44, which would mean an ammo bag contains less than 151.7 points. The earlier tests with a pile of 3 bags showed at least 153.7 per bag. Perhaps refilling from multiple sources very slightly increases the point value of each bag?
It does not matter how far you stand from the bag/crate.
A light crate/box is about 3 bags. A heavy crate is about 6 bags. Request rifleman and throw more bags!
EDIT: Insurgent sapper tank mines (that can also be used in remote mode) use up two slots of ammo per mine! Same goes for the insurgent kit that has a booby trap grenade - two slots for one grenade. Though oddly the insurgent sniper, which has 2 of the same booby trap grenade, only uses one slot's worth of ammo for all of them.
