This mod takes place during the second battle of Fallujah during the Iraq War
This system is made to be like the more modern games. The M4 here starts out with a full mag and 150 extra rounds (during the video it has 90 some odd rounds left because I was testing the system) and each time you reload it subtracts the necessary rounds from your spare ammunition. It works on the same dynamics as the shotguns do. Sorry for crappy low quality, you can blame Bandicam for that.
nice