Okay go to backlot. Go pick up the R700 on the top roof of the building in the middle of the map.
Then go switch to your pistol or whatever other weapon you have and go pick up the G3 in the construction site.
You will notice it has more shots then the R700.
Switch to the R700 and you will notice it has the same amount of bullets as the G3.
Keep picking up the G3 for a crap load of R700 ammo
Hope you didn't know this one