Ensure that ammo is taken from all available inventories, i.e. taking ammo from the BDU and Rucksack when only viewing the vest inventory.
Additionally, add a gameplay option for automatically dropping unusable ammo. This function in ArmA 2 was double edged. It was nice to quickly empty my inventory when grabbing a new weapon, but was a pain when trying to carry extra ammo for a team mate e.g. MG belts or 40mm grenades for other team members.