In most missons there is alot of terroists which
means alot of shooting. Ammo boxes are scarce in
all missions and in some they aren't even there.
To help save ammo, when you have a gun (something
other then sniper rifle, machine gun or MP5) make
sure it is in semi auto fire mode and not 3 round
burst or full auto. It takes one shot to kill and
this will save you ammo for you can choose how
many bullets to send at them unlike full auto.