In real life there are many types of ammo that doen't appear in game, my suggestion is to add them to make players explore more the toxic zones and create more dynamism in the game, some of the ammos i though about are:

• 12 gauge Birdshot - spawn in hunting locations (less damage than the Buckshot)
• 7.62/.308/5.56/5.45 AP rounds - spawn only in toxic zones in stacks of at max 5 (same damage as the normal ones, but have more penetration to vests)
• 9mm/.45 hollow point bullets - spawn at police crashes (50% more damage than the normal ones but less penetration to vests)
• Poiseness arrows - mix the arrow with berries found in bushes (same damage as the normal ones but gives the same/similar sickness as pox)