Extra Magazine allows you to increase the total ammo capacity of your teammates (or yourself) by an extra magazine's worth. Subsequent resupply by Soldiers will fill up ammo to the increased limit.
This increases the ammunition that resupplying provides by a further magazine's worth. This capacity increase remains until the player dies. Ammo resupplies will refill to this new maximum, a feature that is good for weapons with low ammo counts, such as the Belgo.