I ran across an idea (Mods I'll let you decide where this should go). After you complete all your missions, and you are battling the gangs around Los Santos, Try this idea out, Go to the Military installation near Las Venturas and see if you can steal a tank from there. Yes you will have LEO's trying to catch you, but, do a save at the abandoned airfield and drive back to Los Santos in the tank. Shoot like normal to provoke your wars but get in the tank and shoot at the gangs.
This idea will save you from having to use all your ammo in your regular firearms. It is kinda tricky, but, it can be done. It'll also save you from being overwhelmed by the gang members you are fighting. Take this for what it's worth.