I don't usually do ammo runs in SF or LV. I know where to find the weapons I want, and can buy most of them when needed. I frequently use the Ammu-nation glitch to stock up on ammo for my main weapons.
Ammu-nation Glitch
Each time CJ enters and exits the ammu-nation challenge with an M4, Combat Shotgun, Sawn-off Shotgun, SMG, Tec-9, Desert Eagle, or Silenced Pistol in inventory, the ammo in those weapons will double. The glitch might stop working for the pistols if you complete the pistol challenge and not the AK-47 challenge available when the desert is unlocked. The ammu-nation challenge will eat all of the ammo in a regular shotgun - which might be good or bad depending on the situation.
It's pretty easy to glitch the ammo so full that it becomes infinite. But then it is hard to tell when CJ needs to reload because the counter dissappears. It's possible to get rid of an infinite ammo Tec-9 or Micro-SMG by entering the Bloodbowl stadium event in SF. But an infinite ammo M4 is pretty difficult to get rid of if CJ has been dating the girlfriends.