Here is how the ammo system works:

Imagine that each piece of expendable equipment (magazines, C4, grenades, field dressings, smoke grenades, ammo bags, defibrillator charges, etc.) you carry has an individual "ammo bar". When this ammo bar is full, you have the maximum amount of equipment (e.g. nine magazines or one C4 charge); and when the ammo bar is empty, you have no equipment. So if you expend one magazine, then your primary weapon ammo bar will be 8/9ths full, but if you expend one C4 charge, then your C4 ammo bar will be completely empty. Also, all ammo bars recharge (when near an ammo source) at the same rate, so it takes just as long to reload nine magazines as it does to reload one C4 charge, or one field dressing (for most kits).

Now, when you are next to an ammo source, all of your ammo bars recharge simultaneously. But if you move off the ammo source before you have reloaded any ammo, the bars will reset. Even though your ammo bars reset to their pre-charging levels, the ammo used from the source does not reset. This means it is possible to expend an ammo source (such as an ammo bag) without ever getting any equipment off it. The ammo bag itself has enough ammo to fully recharge ~2.1 pieces of equipment. So, if you are the only one using it, you could get about 20 magazines, nine grenades, two field dressings, or two C4 charges. But if you have two pieces of ammo depleted equipment, then (since they all reload simultaneously) you could only get one full reload of each. And if you have three or more pieces of depleted equipment, say one LAT, one smoke grenade, and a field dressing, then you would use up all of the ammo bags ammo before you had reloaded any of your equipment (and then your ammo bars would reset to zero, so you would have wasted the ammo bag).

That probably wasn't particularly enlightening (or understandable) so here are a few rules of thumb:

• Never have more than one person use an ammo bag at a time.
• Rearming an ammo bag cuts the remaining ammo in half (enough to reload one piece of equipment).
• Do not use ammo bags casually, they should be reserved for emergency situations (fully depleted primary weapon, fully depleted defibrillator, extra LAT/HAT needed to destroy an enemy vehicle).
• Only rearm an ammo bag when the ammo needed is negligible: two magazines, an extra grenade, a couple defib charges.