When reloading a weapon that still holds a cartridge/a shell in the chamber the amount of ammunition loaded with the next magazine has to be increased by 1.
For example: EBR with a 20 Round magazine, loaded with exactly 20 rounds.
The player shoots 10 rounds and reloads (with a full magazine from his inventory). The weapon is now loaded with 21 rounds and the player has a spare magazine in his inventory with 9 rounds remaining.
Battlefield 3 can do it, Call of Duty can do it, I believe you can do it aswell.