Broken/More Ammunition Glitch
Picking up a assault rifle or carbine rifle in GTA IV will raise ammunition for 30 rounds.
After collecting the carbine rifle in the abandoned warehouse in Lompoc Avenue in Bohan Industrial for the second occasion this will change: From now on you'll get 80 rounds everytime you swap the gun in this weapon slot from carbine rifle to assault rifle and reverse. This works everywhere in the original game and both addons and is quiet useful for Niko as he has to wait a long time until he can buy this equipment.
Maybe this is a glitch or one of the game's features...