The 20mm Special HE is made for anti-materiel purposes with the addition of explosion upon hitting targets.
A set of ammo contains 3 bullets. The maximum ammo capacity in Original mode is 30, which needs 10 sets of ammo. Each set costs $100 and the total cost is $1000. In Zombie Scenario, buying the whole set can consume up to $20,000!
- 20mm caliber at Wikipedia.